De moivre’s theorem and its applications - class-XII
Description: de moivre’s theorem and its applications | |
Number of Questions: 38 | |
Created by: Rekha Rai | |
Tags: maths demoivre's theorem complex numbers |
If $iz^4 + 1 = 0$, then z can take the value
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$\displaystyle \frac{1 + i}{\sqrt 2}$
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$\cos \displaystyle \frac{\pi}{8} + i \sin \frac{\pi}{8}$
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$\displaystyle \frac{1}{4 i}$
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$i$
The product of the values of $\displaystyle{\left[ {\cos {\pi \over 3} + i\sin {\pi \over 3}} \right]^{{3 \over 4}}}$ is
-
$-1$
-
$1$
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$i$
-
$-i$
Given: $\displaystyle{\left[ {\cos {\pi \over 3} + i\sin {\pi \over 3}} \right]^{{3 \over 4}}}$
$=[e^{i(\pi/3)}]^{(3/4)}=e^{i\pi(1/3)(3/4)}=e^{4\pi i}=cos4\pi+isin{4\pi}=1-0i=1$
Number of integral values of n for which the quantity ${n+i}^{4}$ where ${i}^{2}=-1$, is an integer is
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$1$
-
$2$
-
$3$
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Infinite
Simply $n+i^{4}$ = $n+1$ which is integer for every integral value of n .
De Moivre's theorem
$(\cos\theta +i\sin \theta )=\cos n\theta $ if n is an integer and $\cos n\theta +i \sin n\theta $ is one of the values of $(\cos\theta +i\sin\theta )^{n}$, if n is a fraction.
Corollary : The q values of ($(\cos\theta +i\sin\theta )^{\frac{1}{q}}$ are obtained from
cos $\frac{2n\pi +\theta }{q}+i\sin\frac{2n\pi +\theta }{q}$ by putting n = 0, 1, 2, ..., (q - 1).
-
Both are correct
-
Only first statement is true.
-
Only second ststement is true
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None
If $a = {\mathop{\rm cis}\nolimits} \alpha ,b = cis\beta ,c = cis\gamma $ then $\dfrac{{{a^3}{b^3}}}{{{c^2}}} = $
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$cis(3\alpha + 3\beta + 2\gamma )$
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$cis(3\alpha + 3\beta - 2\gamma )$
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$cis( - 3\alpha - 3\beta + 2\gamma )$
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$cis(3\alpha - 3\beta + 2\gamma )$
$a=cis\alpha=Cos\alpha+iSin\alpha= \ e^{i\alpha}$
$b=cis\beta=Cos\beta+iSin\beta= \ e^{i\beta}$
$a=cis\gamma=Cos\gamma+iSin\gamma= \ e^{i\gamma}$
$\therefore \dfrac{a^3b^3}{c^2}=\dfrac{({e^{i\alpha}})^3({e^{i\beta}})^3}{({e^{i\gamma}})^2}$
$=\dfrac{e^{3i\alpha}e^{3i\beta}} {e^{2i\gamma}}=\ e^{i(3\alpha+3\beta-2\gamma)}$
$=Cos(3\alpha+3\beta-2\gamma)+iSin(3\alpha+3\beta-2\gamma)$
$=cis(3\alpha+3\beta-2\gamma)$
If $a=\cos { \left( \cfrac { 8\pi }{ 11 } \right) } +i\sin { \left( \cfrac { 8\pi }{ 11 } \right) } $, then $Re(a+{a}^{2}+{a}^{3}+{a}^{4}+{a}^{5})=$
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$0$
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$-\cfrac{1}{2}$
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$\cfrac{1}{2}$
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$1$
For ${ Z } _{ 1 }=\sqrt [ 6 ]{ \dfrac { 1-i }{ 1+i\sqrt { 3 } } } $, ${ Z } _{ 2 }=\sqrt [ 6 ]{ \dfrac { 1-i }{ \sqrt { 3 } +i } } $, ${ Z } _{ 3 }=\sqrt [ 6 ]{ \dfrac { 1+i }{ \sqrt { 3 } -i } } $ which of the following holds goods?
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$\sum { { \left| { Z } _{ 1 } \right| }^{ 2 } } =\dfrac { 3 }{ 2 } $
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${ \left| { Z } _{ 1 } \right| }^{ 4 }+{ \left| { Z } _{ 2 } \right| }^{ 4 }={ \left| { Z } _{ 3 } \right| }^{ -8 }$
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$\sum { { \left| { Z } _{ 1 } \right| }^{ 3 }+{ \left| { Z } _{ 2 } \right| }^{ 3 }={ \left| { Z } _{ 3 } \right| }^{ -6 } } $
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$\ \ \ { \left| { Z } _{ 1 } \right| }^{ 4 }+{ \left| { Z } _{ 2 } \right| }^{ 4 }={ \left| { Z } _{ 3 } \right| }^{ 8 }$
Given z is a complex number with modulus 1. Then the equation $\left[\dfrac{(1+ia)}{(1-ia)}\right]^4$ = z has
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all roots real and distinct
-
two real and one imaginary
-
three roots real and one imaginary
-
one root real and three imaginary
$\displaystyle { \left( \frac { 1+ia }{ 1-ia } \right) }^{ 4 }=z\quad $ ...(1)
$\displaystyle & \quad \left| z \right| =1$
$\displaystyle z=cisA=\cos { A } +i\sin { A } $
substitute z in equation (1)
$\displaystyle { \left( \frac { 1+ia }{ 1-ia } \right) }={ cisA }^{ \frac { 1 }{ 4 } }=cis\frac { 2k\pi +A }{ 4 } $ ...{De Moivre's Theorem}
where $k=0,1,2,3$
Let $\displaystyle B=\frac { 2k\pi +A }{ 4 } $
$\displaystyle \Longrightarrow ia=\frac { -1+cisB }{ 1+cisB } =\frac { \sin { \frac { B }{ 2 } \left( i\cos { \frac { B }{ 2 } } -\sin { \frac { B }{ 2 } } \right) } }{ \cos { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } } $
$\displaystyle \Longrightarrow ia=\frac { i\sin { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } }{ \cos { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } } $
$\displaystyle \Longrightarrow a=\tan { \frac { B }{ 2 } } $
Therefore roots are real and distinct.
Ans: A
If $\sqrt{5 - 12i} + \sqrt{-5 - 12i} = z$, then principal value of arg z can be
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$-\displaystyle\frac{\pi}{4}$
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$\displaystyle\frac{\pi}{4}$
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$-\displaystyle\frac{3\pi}{4}$
-
$\displaystyle\frac{3\pi}{4}$
Dividing and multiplying by $\sqrt{13}$
$=\sqrt{13}[(\dfrac{5}{13}-\dfrac{12i}{13})^{\dfrac{1}{2}}+(-\dfrac{5}{13}-\dfrac{12i}{13})^{\dfrac{1}{2}}]$
$=\sqrt{13}[e^{i\dfrac{-\theta}{2}}+e^{i\dfrac{\theta-\pi}{2}}]$
$=\sqrt{13}[cos\dfrac{\theta}{2}-isin\dfrac{\theta}{2}+sin\dfrac{\theta}{2}-icos\frac{\theta}{2}]$
$=\sqrt{13}[cos\dfrac{\theta}{2}+sin\dfrac{\theta}{2}-i(sin\dfrac{\theta}{2}+cos\dfrac{\theta}{2})]$
$=z$
Here $\theta=sin^{-1}(\dfrac{12}{13})$
Hence
$|Re(z)|=|Im(z)|$
Hence argument of $Z$ is in the form of $\dfrac{2n-1(\pi)}{4}$ $n\epsilon::Integers$
The value of $\displaystyle { \left( \frac { 1+i }{ \sqrt { 2 } } \right) }^{ 8 }+{ \left( \frac { 1-i }{ \sqrt { 2 } } \right) }^{ 8 }$ is equal to
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$4$
-
$6$
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$8$
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$2$
We have $\displaystyle { \left( \frac { 1+i }{ \sqrt { 2 } } \right) }^{ 8 }+{ \left( \frac { 1-i }{ \sqrt { 2 } } \right) }^{ 8 }$
The number of solutions of equation $z^{10}-z^{5}+1=0$ are
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only two solution
-
No solution
-
only five solution
-
exactly 10
${ z }^{ 10 }-{ z }^{ 5 }+1=0$
Let ${ z }^{ 5 }=w$
$\Rightarrow { w }^{ 2 }-w+1=0$
$\Rightarrow w=\frac { 1\pm \sqrt { 3 } }{ 2 } =\cos { \frac { \Pi }{ 3 } } \pm \sin { \frac { \Pi }{ 3 } } =cis\left( \pm \frac { \Pi }{ 3 } \right) $
$\Rightarrow { z }^{ 5 }=cis\left( \pm \frac { \Pi }{ 3 } \right) $
Case 1:
${ z }^{ 5 }=cis\left( \frac { \Pi }{ 3 } \right) $
$\Rightarrow z={ \left( cis\left( \frac { \Pi }{ 3 } \right) \right) }^{ \frac { 1 }{ 5 } }=cis\left( \frac { 2k\Pi +\Pi }{ 15 } \right) \ $ ...{De Moivre's Theorem}
Where k=0,1,2,3,4.
Therefore number of solutions are 5.
Case 2:
${ z }^{ 5 }=cis\left( -\frac { \Pi }{ 3 } \right) $
$\Rightarrow z={ \left( cis\left( -\frac { \Pi }{ 3 } \right) \right) }^{ \frac { 1 }{ 5 } }=cis\left( \frac { 2k\Pi -\Pi }{ 15 } \right) $ ...{De Moivre's Theorem}
Where k=0,1,2,3,4.
Therefore number of solutions are 5.
From case 1 & case 2 total number of solutions of equation ${ z }^{ 10 }-{ z }^{ 5 }+1=0$ are 10.
Ans: D
If $\displaystyle z=1+\cos \frac{2\pi }{3}+i\sin \frac{2\pi }{3}$, then
-
$\displaystyle Re(z^{5})=\frac{\sqrt{3}}{2}$
-
$\displaystyle Re(z^{5})=\frac{1}{2}$
-
$\displaystyle Im(z^{5})=\frac{1}{2}$
-
$\displaystyle Im(z^{5})=\frac{\sqrt{3}}{2}$
$z=1+\cos { \frac { 2\pi }{ 3 } } +i\sin { \frac { 2\pi }{ 3 } } =2\cos ^{ 2 }{ \frac { \pi }{ 3 } } +2i\sin { \frac { \pi }{ 3 } \cos { \frac { \pi }{ 3 } } } $
$\displaystyle \Rightarrow z=2\cos { \frac { \pi }{ 3 } } \left( \cos { \frac { \pi }{ 3 } } +i\sin { \frac { \pi }{ 3 } } \right) =\cos { \frac { \pi }{ 3 } } +i\sin { \frac { \pi }{ 3 } } $
$\displaystyle \Rightarrow { z }^{ 5 }={ \left( \cos { \frac { \pi }{ 3 } } +i\sin { \frac { \pi }{ 3 } } \right) }^{ 5 }=\cos { \frac { 5\pi }{ 3 } } +i\sin { \frac { 5\pi }{ 3 } } $ ...{De Moivre's Theorem}
$\displaystyle \Rightarrow { z }^{ 5 }=\frac { 1-i\sqrt { 3 } }{ 2 } $
$\displaystyle \therefore \quad Re\left( { z }^{ 5 } \right) =\frac { 1 }{ 2 } \quad & \quad Im\left( { z }^{ 5 } \right) =\frac { -\sqrt { 3 } }{ 2 } $
Hence, option B is correct.
Construct an equation whose roots are $n^{th}$ powers of the roots of the equation $\displaystyle x^{2}-2x\cos \theta +1= 0.$
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$\displaystyle x^{2}-2n\cos n\theta x+1= 0$
-
$\displaystyle x^{2}-2n\cos \theta x+1= 0$
-
$\displaystyle x^{2}-2\cos n\theta x+1= 0$
-
$\displaystyle x^{2}-2\cos ^{n}\theta x+1= 0$
We know, $\displaystyle \alpha = \cos \theta +i\sin \theta , \beta = \cos \theta -i\sin \theta $
$\displaystyle \alpha ^{n}= \cos n\theta +i\sin n\theta ,$
$\displaystyle \beta ^{n}= \cos n\theta -i\sin n\theta $
$\displaystyle S= 2\cos n\theta , P= 1 \therefore x^{2}-Sx+P= 0$
or $\displaystyle x^{2}-2\cos n\theta x+1= 0$ is the required equation.
Ans: C
If $z = \left(\displaystyle\frac{\sqrt3}{2} + \displaystyle\frac{i}{2}\right)^{2009}+\left(\displaystyle\frac{\sqrt3}{2} - \displaystyle\frac{i}{2}\right)^{2009}$, then
-
$Im(z) = 0$
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$Re(z) > 0$
-
$Im(z) > 0$
-
$Re(z) < 0, Im(z) > 0$
As we know that,
$\dfrac { \sqrt { 3 } }{ 2 } +\dfrac { i }{ 2 } =\cos { \dfrac { \pi }{ 6 } } +i\sin { \dfrac { \pi }{ 6 } } $
and $\dfrac { \sqrt { 3 } }{ 2 } -\dfrac { i }{ 2 } =\cos { \dfrac { \pi }{ 6 } } -i\sin { \dfrac { \pi }{ 6 } } $
$z=\left( \dfrac { \sqrt { 3 } }{ 2 } +\dfrac { i }{ 2 } \right) ^{ 2009 }+\left( \dfrac { \sqrt { 3 } }{ 2 } -\dfrac { i }{ 2 } \right) ^{ 2009 }$
$\Rightarrow z=\left( \cos { \dfrac { \pi }{ 6 } } +i\sin { \dfrac { \pi }{ 6 } } \right) ^{ 2009 }+\left( \cos { \dfrac { \pi }{ 6 } } -i\sin { \dfrac { \pi }{ 6 } } \right) ^{ 2009 }$ ......{ De Moivre's Theorem}
$\Rightarrow z=\cos { \dfrac { 2009\pi }{ 6 } } +i\sin { \dfrac { 2009\pi }{ 6 } } +\cos { \dfrac { 2009\pi }{ 6 } } -i\sin { \dfrac { 2009\pi }{ 6 } } $
$\Rightarrow z=2\cos { \dfrac { 2009\pi }{ 6 } } $
Therefore, $Im(z)=0$
Ans: B
The roots of $\displaystyle \left ( -64a^{4} \right )^{\tfrac14}$ are
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$\displaystyle \pm 2a\left ( 1\pm i \right ).$
-
$\displaystyle \pm a\left ( 1\pm i \right ).$
-
$\displaystyle \pm 2a\left ( 1\pm 2i \right ).$
-
$\displaystyle \pm a\left ( 1\pm 2i \right ).$
The value of $(iz+z^5+z^8)$ when $z=\dfrac{\sqrt{3}+i}{2}$ is?
-
$0$
-
$-1$
-
$\dfrac{-\sqrt{3}+i}{2}$
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$z$
$z=\dfrac{\sqrt{3}+i}{2}=\cos \dfrac{\pi}{6}+i\sin\dfrac{\pi}{6}=cis\dfrac{\pi}{6}$
The value of $\displaystyle \left ( \sin \frac{\pi }{8}+i\cos \frac{\pi }{8} \right )^{8}$
-
-1
-
1
-
0
-
None of these
$z={ \left( \sin { \frac { \pi }{ 8 } +i } \cos { \frac { \pi }{ 8 } } \right) }^{ 8 }={ \left[ i\left( \cos { \frac { \pi }{ 8 } -i\sin { \frac { \pi }{ 8 } } } \right) \right] }]^8$
...{$\because \quad { i }^{ 8 }=1$}
$\Rightarrow z=\cos { \pi -i\sin { \pi } } =-1$ ...{De Moivre's Theorem}
Hence, option 'A' is correct.
If $z=\cos 2\theta +i\sin 2\theta $ then which is correct
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$\displaystyle \sum _{r=0}^{n}C _{r}\cos2r\theta =2^{n} \cos ^{n}\theta \cos n\theta $
-
$\displaystyle \sum _{r=1}^{n}C _{r}\cos2r\theta =2^{n} \sin ^{n}\theta \cos n\theta $
-
$\sum _{ r=0 }^{ n } C _{ r }\sin 2r\theta =2^{ n }\cos ^{ n } \theta \sin n\theta $
-
$\displaystyle \sum _{r=0}^{n}C _{r}\sin2r\theta =2^{n} \sin ^{n}\theta \sin n\theta $
By Binomial Theorem
${ \left( 1+z \right) }^{ n }={ C } _{ 0 }+{ C } _{ 1 }z+{ C } _{ 2 }{ z }^{ 2 }+{ C } _{ 3 }{ z }^{ \ 3 }+....+{ C } _{ n }{ z }^{ n }=\sum _{ r=0 }^{ n }{ { C } _{ r }{ z }^{ r } } $ ...(1)
Substituting $z=\cos { 2\theta } +i\sin { 2\theta } $ in eq. (1), we get
${ \left( 1+\cos { 2\theta } +i\sin { 2\theta } \right) }^{ n }=\sum _{ r=0 }^{ n }{ { C } _{ r }\left( \cos { 2\theta } +i\sin { 2\theta } \right) ^{ r } } $
$\Rightarrow \sum _{ r=0 }^{ n }{ { C } _{ r }\left( \cos { 2r\theta } +i\sin { 2r\theta } \right) }$ ...{De Moivre's Theorem}
$\Rightarrow { \left[ 2\cos { \theta } \left( \cos { \theta } +i\sin { \theta } \right) \right] }^{ n }=\sum _{ r=0 }^{ n }{ { C } _{ r }\cos { 2r\theta } } +i\sum _{ r=0 }^{ n }{ { C } _{ r }\sin { 2r\theta } } $
$\Rightarrow \sum _{ r=0 }^{ n }{ { C } _{ r }\cos { 2r\theta } } +i\sum _{ r=0 }^{ n }{ { C } _{ r }\sin { 2r\theta } } ={ 2 }^{ n }\cos ^{ n }{ \theta } { \left( \cos { n\theta } +i\sin { n\theta } \right) }$ ...{De Moivre's Theorem}
$\Rightarrow \sum _{ r=0 }^{ n }{ { C } _{ r }\cos { 2r\theta } } +i\left( \sum _{ r=0 }^{ n }{ { C } _{ r }\sin { 2r\theta } } \right) ={ 2 }^{ n }\cos ^{ n }{ \theta } \cos { n\theta } +i\left( { 2 }^{ n }\cos ^{ n }{ \theta } \sin { n\theta } \right) $
On comparing real and Imaginary parts, we get
$\sum _{ r=0 }^{ n }{ { C } _{ r }\cos { 2r\theta } } ={ 2 }^{ n }\cos ^{ n }{ \theta } \cos { n\theta } \quad & \quad \sum _{ r=0 }^{ n }{ { C } _{ r }\sin { 2r\theta } } ={ 2 }^{ n }\cos ^{ n }{ \theta } \sin { n\theta } $
Hence, option 'A' and 'C' are correct.
Put in the form A +iB
-
$\displaystyle\cos 47\theta +i\sin47\theta.$
-
$\displaystyle\cos 47\theta -i\sin47\theta.$
-
$\displaystyle\cos 41\theta +i\sin41\theta.$
-
$\displaystyle\cos 41\theta -i\sin41\theta.$
Using De-Moivre's Theorem, the given expression
$\displaystyle = \frac{\left ( \cos \theta -i\sin \theta \right )^{14}\left ( \cos \theta +i\sin \theta \right )^{-15}}{\left ( \cos \theta +i\sin \theta \right )^{48}\left ( \cos \theta +i\sin \theta \right )^{-30}}$
$\displaystyle=\frac{\left ( e^{i\theta } \right )^{-29}}{\left ( e^{i\theta } \right )^{18}}=\left ( e^{i\theta } \right )^{-47}$
$\displaystyle=\left ( \cos \theta +i\sin \theta \right )-^{47}=\cos 47\theta -\sin47\theta.$
Ans: B
If $z = \left(\displaystyle\frac{\sqrt3}{2}+\displaystyle\frac{i}{2}\right)^5 + \left(\displaystyle\frac{\sqrt3}{2}-\displaystyle\frac{i}{2}\right)^5,$ then
-
$Re(z) = 0$
-
$Im(z) = 0$
-
$Re(z) > 0, \space Im(z) > 0$
-
$Re(z) > 0, \space Im(z) < 0$
As we know that,
$\dfrac { \sqrt { 3 } }{ 2 } +\dfrac { i }{ 2 } =\cos { \dfrac { \pi }{ 6 } } +i\sin { \dfrac { \pi }{ 6 } } $
and $\dfrac { \sqrt { 3 } }{ 2 } -\dfrac { i }{ 2 } =\cos { \dfrac { \pi }{ 6 } } -i\sin { \dfrac { \pi }{ 6 } } $
$z=\left( \dfrac { \sqrt { 3 } }{ 2 } +\dfrac { i }{ 2 } \right) ^{ 5 }+\left( \dfrac { \sqrt { 3 } }{ 2 } -\dfrac { i }{ 2 } \right) ^{ 5 }$
$\Rightarrow z=\left( \cos { \dfrac { \pi }{ 6 } } +i\sin { \dfrac { \pi }{ 6 } } \right) ^{ 5 }+\left( \cos { \dfrac { \pi }{ 6 } } -i\sin { \dfrac { \pi }{ 6 } } \right) ^{ 5 }$ ......{ De Moivre's Theorem}
$\Rightarrow z=\cos { \dfrac { 5\pi }{ 6 } } +i\sin { \dfrac { 5\pi }{ 6 } } +\cos { \dfrac { 5\pi }{ 6 } } -i\sin { \dfrac { 5\pi }{ 6 } } $
$\Rightarrow z=-\dfrac { \sqrt { 3 } }{ 2 } $
Therefore, $Im(z)=0$
Ans: B
If $z + z^{-1} = 1$, then $z^{100} + z^{-100}$ is equal to
-
$i$
-
$-i$
-
$1$
-
$-1$
The modulus and amplitude of the complex number $[e^{3-i \tfrac{\pi}{4}}]^3$ are respectively.
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$e^9, \dfrac{\pi}{2}$
-
$e^9, \dfrac{-\pi}{2}$
-
$e^6, \dfrac{-3\pi}{4}$
-
$e^9, \dfrac{-3\pi}{4}$
$z=(e^{3-\tfrac{i\pi}{4}})^{3}$
$=(e^{3}.e^{-\tfrac{i\pi}{4}})^{3}$
$=e^{9}.e^{-\tfrac{3i\pi}{4}}$
$=|z|e^{i\arg(z)}$
By comparing RHS and LHS we get
$|z|=e^{9}$ and $\arg(z)=\dfrac{-3\pi}{4}$.
If $\displaystyle\alpha =\cos { \left( \frac { 8\pi }{ 11 } \right) } +i\sin { \left( \frac { 8\pi }{ 11 } \right) } ,$ then $Re\left( \alpha +{ \alpha }^{ 2 }+{ \alpha }^{ 3 }+{ \alpha }^{ 4 }+{ \alpha }^{ 5 } \right) $ is equal to
-
$\displaystyle\frac { 1 }{ 2 } $
-
$\displaystyle-\frac { 1 }{ 2 } $
-
$0$
-
None of these
$\displaystyle\alpha =\cos { \left( \frac { 8\pi }{ 11 } \right) } +i\sin { \left( \frac { 8\pi }{ 11 } \right) } $
If $x = \cos \theta + i \sin \theta$ the value of $x^n + \dfrac{1}{x^n}$ is
-
$2 \cos n \theta$
-
$2 i \sin n \theta$
-
$2 \sin n \theta$
-
$2 i \cos n \theta$
If $\alpha, \beta$ are the roots of the equation $u^2-2u+2=0$ and if $\cot\theta=x+1$, then $[(x+\alpha)^n-(x+\beta)^m]/[\alpha-\beta]$ is equal to
-
$\displaystyle \frac {\sin n\theta}{\sin^n\theta}$
-
$\displaystyle \frac {\cos n\theta}{\cos^n\theta}$
-
$\displaystyle \frac {\sin n\theta}{\cos^n\theta}$
-
$\displaystyle \frac {\cos n\theta}{\sin^n\theta}$
${ u }^{ 2 }-2u+2=0$
$\Longrightarrow \quad u=1\pm i$
So,$\alpha =1+i\quad and\quad \beta =1-i$
Now given that,$x=\cot { \theta } -1$
so,$\displaystyle \frac { { (x+\alpha ) }^{ n }-{ (x+\beta ) }^{ n } }{ \alpha -\beta } =\frac { { (\cot { \theta } -1+1+i) }^{ n }-{ (\cot { \theta } -1 }+1-i)^{ n } }{ 2i } \ \ $
$=\displaystyle \frac { { (\cot { \theta } +i) }^{ n }-(\cot { \theta } -i)^{ n } }{ 2i } =\frac { { (\cos { \theta } +i\sin { \theta } ) }^{ n }-{ (\cos { \theta } -\sin { \theta } ) }^{ n } }{ ({ \sin { \theta } })^{ n }(2i) } \ \ $
$=\displaystyle \frac { { e }^{ (in\theta ) }-{ e }^{ -(in\theta ) } }{ ({ \sin { \theta ) } }^{ n }2i } \ \ $
=$\displaystyle \frac { (\cos { (n\theta ) } +i\sin { (n\theta )) } -(\cos { (n\theta ) } -i\sin { (n\theta )) } }{ ({ \sin { \theta ) } }^{ n }2i } =\frac { \sin { (n\theta ) } }{ { (\sin { \theta ) } }^{ n } } \ \ $
If $z _{1}$ and $\bar {z} _{1}$ represent adjacent of a regular polygon of $n$ sides with centre at the origin & if $\dfrac{Im\ z _{1}}{Re\ z _{1}}=\sqrt{2}-1$ then the value of $n$ is equal to:
-
$8$
-
$12$
-
$16$
-
$24$
What is the real part of $(\sin x + i \cos x)^{3}$ where $i = \sqrt {-1}$?
-
$-\cos 3x$
-
$-\sin 3x$
-
$\sin 3x$
-
$\cos 3x$
If $(\cos \theta + i \sin \theta)(\cos 2 \theta
+ i \sin 2 \theta) ... (\cos n \theta + i \sin n \theta) = 1$, then the value of $\theta$ is , $m\in N$
-
$4m\pi$
-
$\displaystyle \frac{2m\pi}{n(n+1)}$
-
$\displaystyle \frac{4m\pi}{n(n+1)}$
-
$\displaystyle \frac{m\pi}{n(n+1)}$
Changing the above expression to Eular's form, we get
$e^{i\theta}e^{2i\theta}e^{3i\theta}...e^{in\theta})=1$
$e^{i(\theta+2\theta+3\theta+...n\theta}=1$
$e^{i\cfrac{n(n+1)}{2}\theta}=e^{2m\pi}$
Therefore, simplifying we get
$\dfrac{n(n+1)}{2}\theta=2m\pi$
$\theta=\dfrac{4m\pi}{n(n+1)}$
Statement 1: The product of all values of $(cos\alpha+i sin \alpha)^{\frac {3}{5}}$ is $cosn 3\alpha+i sin 3\alpha$.
Statement 2: The product of fifth roots of unity is 1.
-
Both the statements are true, and Statement 2 is the correct explanation for Statement 1.
-
Both the statements are true, but Statement 2 is not the correct explanation for Statement 1.
-
Statement 1 is true and Statement 2 is false.
-
Statement 1 is false and Statement 2 is true.
Let
$z=(cos\theta+isin\theta)^{3}$
Taking the fifth root, we get
$z^{\dfrac{1}{5}}=(cos\theta+isin\theta)^{\dfrac{3}{5}}=x$
Now there will be $5$, corresponding values of $x$.
Product if all the $5$ values will be
$x^{5}$
$=(cos\theta+isin\theta)^{3}$
$=cos3\theta+isin3\theta$ ... using De-Moivre's rule.
Now consider $x^{n}=1$
Hence if $n$ is odd, the nth roots of unity will be
$1,a _{1},\overline{a _{1}},a _{2},\overline{a _{2}}....$
Now $a _{1}.\overline{a _{1}}=|a _{1}|^{2}=1$
$a _{2}.\overline{a _{2}}=|a _{2}|^{2}=1$
:
:
Hence one root will be one, and the rest $n-1$ roots will occur in pair with its conjugate.
Hence product will be $1$.
Substituting, $n=5$, we get the roots as
$1,a _{1},\overline{a _{1}},a _{2},\overline{a _{2}}$
Hence product if all the roots will be $1$. And sum of all the roots will be $0$.
Both the statements are true, but Statement 2 is not the correct explanation for Statement 1.
If $z _1$ and $z _2$ are the complex roots of the equation $(x-3)^3+1 = 0$, then $z _1 + z _2$ equals to
-
1
-
3
-
5
-
7
$cis\left( \theta \right) =\cos { \theta } +i\sin { \theta } $
De Moivre's Theorem for fractional power:
${ \left( cis\theta \right) }^{ \frac { 1 }{ n } }=cis\left( \frac { 2k\Pi +\theta }{ n } \right) $
${ \left( x-3 \right) }^{ 3 }+1=0$
$\Longrightarrow x=3+{ \left( cis\left( \Pi \right) \right) }^{ \frac { 1 }{ 3 } }$
$x=3+{ \left( cis\left( \frac { 2k\Pi +\Pi }{ 3 } \right) \right) }$ ...{De Moivre's Theorem}
Where, $k=0,1,2$
for $k=0$,
$x _{ 1 }=3+cis\left( \frac { \Pi }{ 3 } \right)$
for $k=1$,
$x _{ 2 }=3+cis\left( \Pi \right) $
for $k=2,$
$x _{ 3 }=3+cis\left( \frac { 5\Pi }{ 3 } \right) $
$\Longrightarrow { x } _{ 1 }+{ x } _{ 3 }=6+cis\left( \frac { \Pi }{ 3 } \right) +cis\left( \frac { 5\Pi }{ 3 } \right) \ \Longrightarrow { x } _{ 1 }+{ x } _{ 3 }=7$
Ans: D
If $\left ( 2+z \right )^{6}+\left ( 2-z \right )^{6}=0$ and $\omega =\dfrac{2+z}{2-z}$
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$\displaystyle \omega =e^{i}\tfrac{\left (2p+1 \right )\pi }{6},p=0,1,2,3,4,5$
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$\displaystyle z=\frac{2\left ( \omega -1 \right )}{\omega +1}$
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$\displaystyle \omega = ( -1 )^(\frac{1}{6})$
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All of these
$\displaystyle { \left( 2+z \right) }^{ 6 }{ +\left( 2-z \right) }^{ 6 }=0\quad \quad & \quad w=\frac { 2+z }{ 2-z } $
$\displaystyle \Rightarrow { \left( \frac { 2+z }{ 2-z } \right) }^{ 6 }=-1\ \Rightarrow { w }^{ 6 }=-1$
$\displaystyle { \therefore \quad w=\left( -1 \right) }^{ \frac { 1 }{ 6 } }$
$\displaystyle \because \quad \frac { 2+z }{ 2-z } =w\ \Rightarrow 2\left( w-1 \right) =z\left( w+1 \right) $
$\displaystyle \therefore \quad z=\frac { 2\left( w-1 \right) }{ w+1 } $
$\displaystyle { \because \quad w=\left( -1 \right) }^{ \frac { 1 }{ 6 } }$
$\displaystyle w={ \left( \cos { \pi } +i\sin { \pi } \right) }^{ \frac { 1 }{ 6 } }=\cos { \left( \frac { 2p\pi +\pi }{ 6 } \right) +i } \sin { \left( \frac { 2p\pi +\pi }{ 6 } \right) } $ ..{De Moivre's Theorem}
Where$ p=0,1,2,3,4,5.$
$\displaystyle \Rightarrow w={ e }^{ i\frac { \left( 2p+1 \right) \pi }{ 6 } }$
Hence, option 'D' is correct.
Given $z$ is a complex number with modulus $1$. Then the equation $\dfrac{(1+ia)}{(1-ia)}$ = $z$ has
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all roots real and distinct
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two real and one imaginary
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three roots real and one imaginary
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one root real and three imaginary
$\displaystyle { \left( \frac { 1+ia }{ 1-ia } \right) }^{ 4 }=z\quad $ ...(1)
$\displaystyle & \quad \left| z \right| =1$
$\displaystyle z=cisA=\cos { A } +i\sin { A } $
Substitute $z$ in equation (1)
$\displaystyle { \left( \frac { 1+ia }{ 1-ia } \right) }={ cisA }^{ \frac { 1 }{ 4 } }=cis\frac { 2k\pi +A }{ 4 } $ ...{De Moivre's Theorem}
where $ k=0,1,2,3$
Let $\displaystyle B=\frac { 2k\pi +A }{ 4 } $
$\displaystyle \Longrightarrow ia=\frac { -1+cisB }{ 1+cisB } =\frac { \sin { \frac { B }{ 2 } \left( i\cos { \frac { B }{ 2 } } -\sin { \frac { B }{ 2 } } \right) } }{ \cos { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } } $
$\displaystyle \Longrightarrow ia=\frac { i\sin { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } }{ \cos { \frac { B }{ 2 } \left( \cos { \frac { B }{ 2 } } +i\sin { \frac { B }{ 2 } } \right) } } $
$\displaystyle \Longrightarrow a=\tan { \frac { B }{ 2 } } $
Therefore roots are real and distinct.
Ans: A
If n is a natural number$ \ge$ 2, such that $z^n = (z+ 1)^n$, then
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roots of equation lie on a straight line parallel to the y-axis
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roots of equation lie on a straight line parallel to the x-axis
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sum of the real parts of the roots is -[(n-1)/2]
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none of these
$\displaystyle { z }^{ n }={ \left( z+1 \right) }^{ n }$ where, $n\ge 2$ ...(1)
$\displaystyle \Rightarrow { \left( \frac { z+1 }{ z } \right) }^{ n }=1=\cos { 0 } +i\sin { 0 } $
$\displaystyle \Rightarrow \frac { z+1 }{ z } ={ \left( \cos { 0 } +i\sin { 0 } \right) }^{ \frac { 1 }{ n } }$
$\displaystyle \Rightarrow \frac { z+1 }{ z } =\cos { \frac { 2k\Pi }{ n } } +i\sin { \frac { 2k\Pi }{ n } } $ ...{De Moivre's Theorem}
$\displaystyle \Rightarrow z=\frac { -1 }{ 1-\cos { \frac { 2k\Pi }{ n } } -i\sin { \frac { 2k\Pi }{ n } } } \quad =\frac { -1 }{ 2\sin { \frac { k\Pi }{ n } \left( \sin { \frac { k\Pi }{ n } -i } \cos { \frac { k\Pi }{ n } } \right) } } =\frac { -1\left( \sin { \frac { k\Pi }{ n } +i } \cos { \frac { k\Pi }{ n } } \right) }{ 2\sin { \frac { k\Pi }{ n } } } $
$\displaystyle \Rightarrow z=\frac { -\left( 1+i\cot { \frac { k\Pi }{ n } } \right) }{ 2 } $
Where$ k=1,2,3,....,n-1 $ ..{Since at k=0, z is not defined}
$\because \quad Re\left( z \right) $ is constant.
Therfore roots of ${ z }^{ n }={ \left( z+1 \right) }^{ n }$ lie on straight line parellel to y-axis.
$\displaystyle \because \quad z=\frac { -\left( 1+i\cot { \frac { k\Pi }{ n } } \right) }{ 2 } $ and $k=1,2,3,....,(n-1)$
Sum of $\displaystyle Re\left( z \right) $= $-\frac { \left( n-1 \right) }{ 2 } $.
Ans: A,C
For positive integers $\displaystyle n _{1}$ and $\displaystyle n _{2}$ the value of the expression $\displaystyle (1+i)^{n _{1}}+(1+i^{3})^{n _{1}}+(1+i^{5})^{n _{2}}+(1+i^{2})^{n _{2}}$ where
$\displaystyle i= \sqrt{-1}$ is a real number iff
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$\displaystyle n _{1}= n _{2}$
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$\displaystyle n _{2}= n _{2}-1$
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$\displaystyle n _{1}= n _{2}+1$
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$\displaystyle \forall n _{1}$ and $\displaystyle n _{2}$
$(1+i)^{n _{1}} = $ $ ^{n _{1}}C _{0} + ^{n _{1}}C _{1} i +^{n _{1}}C _{2} i^2 + ......+^{n _{1}}C _{n _{1}} i^{n _{1}}$ --------(1)
If ${ x }^{ 6 }={ \left( 4-3i \right) }^{ 5 }$, then the product of all of its roots is (where $\displaystyle \theta =-\tan ^{ -1 }{ \frac { 3 }{ 4 } } $)
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${ 5 }^{ 5 }\left( \cos { 5\theta } +i\sin { 5\theta } \right) $
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$-{ 5 }^{ 5 }\left( \cos { 5\theta } +i\sin { 5\theta } \right) $
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${ 5 }^{ 5 }\left( \cos { 5\theta } -i\sin { 5\theta } \right) $
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$-{ 5 }^{ 5 }\left( \cos { 5\theta } -i\sin { 5\theta } \right) $
$\displaystyle { x }^{ 6 }={ \left( 4-3i \right) }^{ 5 }\Rightarrow { x }^{ 6 }={ 5 }^{ 6 }\left( \frac { 4 }{ 5 } -\frac { 3i }{ 5 } \right) ={ 5 }^{ 5 }{ \left( \cos { \theta } +i\sin { \theta } \right) }^{ 5 }$
If $C _{o},C _{1},C _{2}...C _{n}$ are the Binomial coefficient in the expansion of $\left ( 1+x \right )^{n}$ then which is not correct
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$C _{0}-C _{2}+C _{4}-C _{6}+...=2\tfrac{n}{2}\cos \frac{n\pi }{4}$
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$C _{1}-C _{3}+C _{5}+...=2\tfrac{n}{2}\sin \frac{n\pi }{4}$
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$C _{1}+C _{5}+C _{9}+C _{13}+...=\tfrac{1}{2}\left ( 2^{n-1}+2\tfrac{n}{2}\sin \frac{n\pi }{4} \right )$
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None of these
By Binomial theorem
${ \left( 1+x \right) }^{ n }={ C } _{ 0 }+{ C } _{ 1 }x+{ C } _{ 2 }{ x }^{ 2 }+{ C } _{ 3 }{ x }^{ \ 3 }+{ C } _{ 4 }{ x }^{ 4 }+...$ ...(1)
Substitute $x=i$ in equation (1), we get
$\Rightarrow { \left( 1+i \right) }^{ n }={ C } _{ 0 }+{ C } _{ 1 }i-{ C } _{ 2 }-{ C } _{ 3 }i+{ C } _{ 4 }+.....$ ...(2)
Substitute $x=-i$ in equation (1), we get
${ \left( 1-i \right) }^{ n }={ C } _{ 0 }-{ C } _{ 1 }i-{ C } _{ 2 }+{ C } _{ 3 }i+{ C } _{ 4 }+.....$ ...(3)
Adding (2) & (3), we have
$\Rightarrow 2\left( { C } _{ 0 }{ -C } _{ 2 }{ +C } _{ 4 }+.... \right) ={ \left( 1+i \right) }^{ n }+{ \left( 1-i \right) }^{ n }$
$\Rightarrow { C } _{ 0 }{ -C } _{ 2 }{ +C } _{ 4 }+....=\dfrac { { \left( 1+i \right) }^{ n }+{ \left( 1-i \right) }^{ n } }{ 2 } ={ 2 }^{ \dfrac { n }{ 2 } }\left( \dfrac { { \left( \cos { \dfrac { \pi }{ 4 } } +i\sin { \dfrac { \pi }{ 4 } } \right) }^{ n }+{ \left( \cos { \dfrac { \pi }{ 4 } } -i\sin { \dfrac { \pi }{ 4 } } \right) }^{ n } }{ 2 } \right) $
$\Rightarrow { C } _{ 0 }{ -C } _{ 2 }{ +C } _{ 4 }+....={ 2 }^{ \dfrac { n }{ 2 } }\left( \dfrac { \cos { \dfrac { n\pi }{ 4 } } +i\sin { \dfrac { n\pi }{ 4 } +\cos { \dfrac { n\pi }{ 4 } } -i\sin { \dfrac { n\pi }{ 4 } } } }{ 2 } \right) $
$\Rightarrow { C } _{ 0 }{ -C } _{ 2 }{ +C } _{ 4 }+....={ 2 }^{ \dfrac { n }{ 2 } }\cos { \dfrac { n\pi }{ 4 } } $
Subtracting (2) & (3), we have
$\Rightarrow 2i\left( { C } _{ 1 }{ -C } _{ 3 }{ +C } _{ 5 }+.... \right) ={ \left( 1+i \right) }^{ n }-{ \left( 1-i \right) }^{ n }$
$\Rightarrow { C } _{ 1 }{ -C } _{ 3 }{ +C } _{ 5 }+....=\dfrac { { \left( 1+i \right) }^{ n }-{ \left( 1-i \right) }^{ n } }{ 2i } ={ 2 }^{ \dfrac { n }{ 2 } }\left( \dfrac { { \left( \cos { \dfrac { \pi }{ 4 } } +i\sin { \dfrac { \pi }{ 4 } } \right) }^{ n }-{ \left( \cos { \dfrac { \pi }{ 4 } } -i\sin { \dfrac { \pi }{ 4 } } \right) }^{ n } }{ 2i } \right) $
$\Rightarrow { C } _{ 1 }{ -C } _{ 3 }{ +C } _{ 5 }+....={ 2 }^{ \dfrac { n }{ 2 } }\left( \dfrac { \cos { \dfrac { n\pi }{ 4 } } +i\sin { \dfrac { n\pi }{ 4 } -\cos { \dfrac { n\pi }{ 4 } } +i\sin { \dfrac { n\pi }{ 4 } } } }{ 2i } \right) $
$\Rightarrow { C } _{ 1 }{ -C } _{ 3 }{ +C } _{ 5 }+.....={ 2 }^{ \dfrac { n }{ 2 } }\sin { \dfrac { n\pi }{ 4 } } $ ...(4)
Substitute $x=1$ in equation (1), we get
$\Rightarrow { C } _{ 0 }+{ C } _{ 1 }{ +C } _{ 2 }+{ C } _{ 3 }+{ C } _{ 4 }+.....\quad ={ 2 }^{ n }$ ...(5)
Substitute $x=-1$ in equation (1), we have
$\ \Rightarrow { C } _{ 0 }{ -C } _{ 1 }{ +C } _{ 2 }-{ C } _{ 3 }+{ C } _{ 4 }+.....\quad =0$ ...(6)
Subtracting (5) & (6), we get
$\Rightarrow 2\left( { C } _{ 1 }+{ C } _{ 3 }+{ C } _{ 5 }+..... \right) ={ 2 }^{ n }$
$\Rightarrow { C } _{ 1 }+{ C } _{ 3 }+{ C } _{ 5 }+.....\quad ={ 2 }^{ n-1 }$ ...(7)
Adding (4) & (7), we get
$\Rightarrow 2\left( { C } _{ 1 }+{ C } _{ 5 }+{ C } _{ 9 }+..... \right) ={ 2 }^{ n-1 }+{ 2 }^{ \dfrac { n }{ 2 } }\sin { \dfrac { n\pi }{ 4 } } $
$\Rightarrow { C } _{ 1 }+{ C } _{ 5 }+{ C } _{ 9 }+.....=\dfrac { 1 }{ 2 } \left( { 2 }^{ n-1 }+{ 2 }^{ \dfrac { n }{ 2 } }\sin { \dfrac { n\pi }{ 4 } } \right) $
Hence, option 'D' is correct.
If $ x+\dfrac{1}{x}=2\cos \theta \ and \ y+\dfrac{1}{y}=2\cos \phi$ then which of the following is not correct?
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$\displaystyle \frac{x}{y} +\frac{y}{x}=2\cos \left ( \theta -\phi \right )$
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$x^{m}y^{n}=\cos \left ( m\theta +n\phi \right )+i\sin \left ( m\theta +n\phi \right )$
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$x^{m}y^{n}+x^{-m}y^{-n}=2\cos \left ( m\theta +n\phi \right )$
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None of these
$x+\dfrac { 1 }{ x } =2\cos { \theta } \quad & \quad y+\dfrac { 1 }{ y } =2\cos { \phi } \ $
$\Rightarrow x=\cos { \theta } +i\sin { \theta } =cis\theta \ \quad & \quad y=\cos { \phi } +i\sin { \phi } =cis\phi \ $
$\dfrac { x }{ y } +\dfrac { y }{ x } =\dfrac { cis\theta }{ cis\phi } +\dfrac { cis\phi }{ cis\theta } =cis\left( \theta -\phi \right) +cis\left( -\theta +\phi \right) $
$\therefore \quad \dfrac { x }{ y } +\dfrac { y }{ x } =2\cos { \left( \theta -\phi \right) } $
${ x }^{ m }{ y }^{ n }={ \left( cis\theta \right) }^{ m }{ \left( cis\phi \right) }^{ n }=\left( cism\theta \right) \left( cisn\phi \right) $ ...{ De Moivre's Theorem}
$\therefore \quad { x }^{ m }{ y }^{ n }=cis\left( m\theta +n\phi \right) =\cos { \left( m\theta +n\phi \right) } +i\sin { \left( m\theta +n\phi \right) } $
${ x }^{ -m }{ y }^{ -n }={ \left( cis\theta \right) }^{ -m }{ \left( cis\phi \right) }^{ -n }=\left( cis\left( -m\theta \right) \right) \left( cis\left( -n\phi \right) \right) \ \therefore \quad { x }^{ -m }{ y }^{ -n }=cis\left( -m\theta -n\phi \right) =\cos { \left( m\theta +n\phi \right) } -i\sin { \left( m\theta +n\phi \right) } \ $
$\therefore \quad { x }^{ m }{ y }^{ n }+{ x }^{ -m }{ y }^{ -n }=2\cos { \left( m\theta +n\phi \right) } $
Let $\mathrm{z}=\cos\theta+\mathrm{i}\sin\theta$. Then the value of $\displaystyle \sum _{\mathrm{m}=1}^{15}{\rm Im}(\mathrm{z}^{2\mathrm{m}-1})$ at $\theta =2^{\mathrm{o}}$ is
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$\displaystyle \frac{1}{\sin 2^{\mathrm{o}}}$
-
$\displaystyle \frac{1}{3\sin 2^{\mathrm{o}}}$
-
$\displaystyle \frac{1}{2\sin 2^{\mathrm{o}}}$
-
$\displaystyle \frac{1}{4\sin 2^{\mathrm{o}}}$
$\mathrm{z}=\cos\theta+\mathrm{i}\sin\theta$
$\Rightarrow z^{2m-1} = \cos(2m-1)\theta+i\sin (2m-1)\theta$
Let $X = \displaystyle \sum _{\mathrm{m}=1}^{15}{\rm Im}(\mathrm{z}^{2\mathrm{m}-1})$
$\therefore \mathrm{X}=\sin\theta+\sin 3\theta+\ldots+\sin 29\theta$
$\Rightarrow 2(\sin\theta)\mathrm{X}=1-\cos 2\theta+\cos 2\theta-\cos 4\theta+\ldots+\cos 28\theta-\cos 30\theta$
$\displaystyle \therefore \mathrm{X}=\frac{1-\cos 30\theta}{2\sin\theta}=\frac{1}{4\sin 2^{\mathrm{o}}}$