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Geometric representation of a complex number - class-XII

Description: geometric representation of a complex number
Number of Questions: 24
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Tags: complex numbers maths complex numbers and quadratic equations complex numbers and linear inequations
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If $z _{1}=8 +4i,\ z _{2}=6+4i$ and $arg \left(\dfrac {z-z _{1}}{z-z _{2}}\right)=\dfrac {\pi}{4}$, then $z$ satisfy 

  1. $|z-7-4i|=1$

  2. $|z-7-5i|=\sqrt {2}$

  3. $|z-4i|=8$

  4. $|z-7i|=\sqrt {18}$


Correct Option: A

In the complex plane, what is the distance of $4-2i$ from the origin?

  1. $2$

  2. $3.46$

  3. $4.47$

  4. $6$

  5. $12$


Correct Option: C
Explanation:

Let $z=4-2i$

In a complex plane, distance of $z$ from any point origin is given by $|z|=\sqrt{(x _1)^2+(y _1)^2}$
$\therefore$ Distance of $z=4-2i$ from origin is given by $|z|=|4-2i|=\sqrt{4^2+(-2)^2}=\sqrt{20}\approx 4.47$
Hence, the answer is $4.47$.

In the complex plane, the number 4 + j3 is located in the

  1. first quadrant

  2. second quadrant

  3. third quadrant

  4. fourth quadrant


Correct Option: A
Explanation:

Since both the real and imaginary parts are positive , 4 + j3 lies in first quadrant of argand/complex plane.

If ${z _1}$ and ${z _2}$ are two non-zero complex number such that $\left| {{{{z _1}} \over {{z _2}}}} \right|$ = 2 and $\arg \left( {{z _1}{z _2}} \right) = {{3\pi } \over 2}$ , then ${{\overline {{z _1}} } \over {{z _2}}}$ is equal to 

  1. 2i

  2. -2

  3. -2i

  4. 2


Correct Option: A
Explanation:

$Let\quad { z } _{ 1 }=2r{ e }^{ i{ \theta  } _{ 1 } },{ z } _{ 2 }=r{ e }^{ i{ \theta  } _{ 2 } }\ arg\left( { z } _{ 1 }{ z } _{ 2 } \right) ={ \theta  } _{ 1 }+{ \theta  } _{ 2 }=\cfrac { 3\pi  }{ 2 } \ \bar { { z } _{ 1 } } =2r{ e }^{ -i{ \theta  } _{ 1 } }\ \cfrac { \bar { { z } _{ 1 } }  }{ { z } _{ 2 } } =\cfrac { 2r{ e }^{ -i{ \theta  } _{ 1 } } }{ r{ e }^{ i{ \theta  } _{ 2 } } } =2r{ e }^{ -i({ \theta  } _{ 1 }+{ \theta  } _{ 2 }) }=2r{ e }^{ -i\cfrac { 3\pi  }{ 2 }  }=2i$

Given $\left| z \right| =4$ and $Argz=\dfrac{5z}{6}$, then $z$ is

  1. $2\sqrt{3}+2i$

  2. $2\sqrt{3}-2i$

  3. $-2\sqrt{3}+2i$

  4. $-\sqrt{3}+i$


Correct Option: C
Explanation:

Given that


$\Rightarrow |z|=4$ and $Arg \space z=\dfrac{5\pi}{6}$

$\Rightarrow $Let $z=x+iy$

$\Rightarrow |z|=4$ 

$\Rightarrow \sqrt{x^2+y^2}=4$

$\Rightarrow {x^2+y^2}=16$                ...$(1)$

$\Rightarrow Arg \space z=\dfrac{5\pi}{6}$

$\Rightarrow \tan ^{-1}(\dfrac{y}{x})=\dfrac{5\pi}{6}$

$\Rightarrow \dfrac{y}{x}=\tan (\dfrac{5\pi}{6})$

$\Rightarrow \dfrac{y}{x}=-\dfrac{1}{\sqrt 3}$
  
$\Rightarrow x^2=3y^2$

Substituting this in $(1)$ we get,

$\Rightarrow x^2+\dfrac{x^2}{3}=16$

$\Rightarrow \dfrac{4x^2}{3}=16$

$\Rightarrow x^2=12$

$\Rightarrow x=\pm 2\sqrt 3$

$\Rightarrow y=\mp 2$

$\Rightarrow \theta$ lies in $2^{nd} Quadrant$

Hence, $\Rightarrow z=-2\sqrt 3+2i$

$|z-4| < |z-2|$ represents the region given by?

  1. $Re(z) > 3$

  2. $Re(z) < 0$

  3. $Re(z) > 2$

  4. None of these


Correct Option: A

If $a, b \notin R$, then $|e^{a + ib}| $ is equal to


  1. $e^a$

  2. $e^b$

  3. $1$

  4. None of these


Correct Option: A
Explanation:

$|e^{a + ib}| = |e^a . e^{ib}|$


            $= |e^a . (\cos b + i \sin b)|$


            $= e^a |(\cos b + i \sin b)|$

            $= e^a . \sqrt{(\cos b)^2 + (\sin b)^2}$

            $= e^a . \sqrt{1}$

            $= e^a$

If $Re(\dfrac{z+2i}{z+4})=0$ then z lies on a circle with center:

  1. (-2,-1)

  2. (-2,1)

  3. (2,-1)

  4. (2,1)


Correct Option: A
Explanation:

Let $z=x+ iy$

Then 

$w=\dfrac{z+2i}{z+4}=\dfrac{x+i(2+y)}{(x+4)+iy}$
Rationalizing we get:

$w=\dfrac{z+2i}{z+4}=\dfrac{x+i(2+y)}{(x+4)-iy}\times \dfrac{x+4-iy}{x+4+iy}=\dfrac{x^2+4x+y^2+2y+i(xy+2x+4y+xy+8)}{(x+4)^2+y^2}$
Since real part of $w$ is 0, hence

$(x+2)^2+(y+1)^2-5=0$

Hence centre of circle is $(-2,-1)$

The argument of the complex number $\sin \dfrac{{6\pi }}{5} + i\left( {1 + \cos \dfrac{{6\pi }}{5}} \right)$ is 

  1. $\dfrac{{6\pi }}{5}$

  2. $\dfrac{{5\pi }}{5}$

  3. $\dfrac{{9\pi }}{10}$

  4. $\dfrac{{7\pi }}{10}$


Correct Option: C
Explanation:
Given that

$\sin { \cfrac { 6\pi  }{ 5 }  } +i\left( 1+\cos { \cfrac { 6\pi  }{ 5 }  }  \right) $

Notice the point $\sin { \left( \cfrac { 6\pi  }{ 5 }  \right)  } +i\left( 1+\cos { \cfrac { 6\pi  }{ 5 }  }  \right) $ or

$-\sin { \left( \cfrac { 6\pi  }{ 5 }  \right)  } +i\left( 1-\cos { \cfrac { \pi  }{ 5 }  }  \right) $ lies in the second quadrant of complex plane hence its argument is given as

$arg\left( z \right) =\pi -\tan ^{ -1 }{ \left| y/x \right|  } \quad $  ($\because z=x+iy$)

$\left( \forall x<0,y\ge 0 \right) $

$arg(z)=\pi -\tan ^{ -1 }{ \left| \cfrac { 1-\cos { \cfrac { \pi  }{ 5 }  }  }{ \sin { \cfrac { \pi  }{ 5 }  }  }  \right|  } $

$=\pi -\tan ^{ -1 }{ \left| \cfrac { 2\sin ^{ 2 }{ \cfrac { \pi  }{ 10 }  }  }{ 2\sin { \cfrac { \pi  }{ 10 }  } \cos { \cfrac { \pi  }{ 10 }  }  }  \right|  } =\pi -\tan ^{ -1 }{ \left| \cfrac { \sin { \cfrac { \pi  }{ 10 }  }  }{ \cos { \cfrac { \pi  }{ 10 }  }  }  \right|  } $

$=\pi -\tan ^{ -1 }{ \left| \tan { \cfrac { \pi  }{ 10 }  }  \right|  } \left( \because \tan ^{ -1 }{ \cfrac { \pi  }{ 10 }  } >1 \right) $

$=\pi -\cfrac { \pi  }{ 10 } \left( \because -\cfrac { \pi  }{ 2 } \le \tan ^{ -1 }{ x } \le \cfrac { \pi  }{ 2 }  \right) $

$arg(z)=\cfrac{9\pi}{10}$

Let $z,w$ be complex numbers such that $\vec {z}+i\vec {w}=$ and $zw=\pi$ Then $arg\ z$ equals

  1. $\dfrac {\pi}{4}$

  2. $\dfrac {5\pi}{4}$

  3. $\dfrac {3\pi}{4}$

  4. $\dfrac {\pi}{2}$


Correct Option: C

Let $A$ and $B$ represent $z _{1}$ and $z _{2}$ in the Argand plane and $z _{1},z _{2}$ be the roots of the equation $z^{2}+pz+q=0$ where $p,q$ are complex numbers. If $O$ is the origin $OA=OB$ and $\angle AOB=\alpha$ then $p^{2}=$

  1. $2q\ \cos \left(\dfrac{\alpha}{2}\right)$

  2. $4q\ \cos \left(\dfrac{\alpha}{2}\right)$

  3. $4q\ \cos^{2} \left(\dfrac{\alpha}{2}\right)$

  4. $4q^{2}\ \cos^{2} \left(\dfrac{\alpha}{2}\right)$


Correct Option: A

Let  $z _ { 1 } , z _ { 2 }$  and  $z _ { 3 }$  represent the vertices  $A, B$  and  $C$  of the triangle  $A B C$  in the argand that  $\left| z _ { 1 } \right| = \left| z _ { 2 } \right| = \left| z _ { 3 } \right| = 5,$  then  $z _ { 1 } \sin 2 A + z _ { 2 } \sin 2 B + z _ { 3 } \sin 2 C = 0.$

  1. True

  2. False


Correct Option: A

If $\sin \frac {6\pi}5+i(1+\cos \frac {6\pi }5)$ then

  1. $|Z|=-2\cos \frac {3\pi}5$

  2. $Arg(Z)=\frac {\pi}5$

  3. $Arg(Z)=\frac {9\pi }{10}$

  4. none of these


Correct Option: C

If Arg $(z + i)\, -$ Arg $(z - i)$ $= \dfrac{\pi}{2}$, then $z$ lies on a ..........

  1. Circle

  2. Line

  3. Coordinate axes

  4. None of these


Correct Option: A
Explanation:
Putting z = x+iy,

${tan}^{-1}\dfrac{y+1}{x}$  -  ${tan}^{-1}\dfrac{y-1}{x}$ = $\pi$/2

$\Rightarrow$ 1 + ($\dfrac{y+1}{x})$($\dfrac{y-1}{x}$) = 0

$\Rightarrow$ $x^2 +  y^2$ = 1

It is a circle of center coinciding with origin and radius 1 units.

Hence, option A is correct.

If $\overline { z } $ lies in the third quadrant then $z$ lies in the

  1. First quadrant

  2. Second quadrant

  3. Third quadrant

  4. Fourth quadrant


Correct Option: B
Explanation:

Fact:  $\overline z$ is image of $z$ in $x$-axis

So if $\overline z$ lies in third quadrant then $z$ will lie in second quadrant 

Let $z _1$ and $z _2$ are two complex numbers such that $(1-i)z _1=2z _2$ and $arg(z _1z _2)=\dfrac{\pi}{2}$ then $arg(z _2)$ is equals to:

  1. $\dfrac{3 \pi}{8}$

  2. $\dfrac{\pi}{8}$

  3. $\dfrac{5 \pi}{8}$

  4. $\dfrac{-7 \pi}{8}$


Correct Option: B
Explanation:

$(1-i)z _1=2z _2$

$\cfrac{z _2}{z _1}=\cfrac{1}{2}-\cfrac{i}{2}$
Let $z _1=r _1e^{i\theta _1}\ and\ z _=r _1r^{\theta _2}$
$arg(\cfrac{z _2}{z _1})=\tan^{-1}\cfrac{-1/2}{1/2}=-\pi/4$
$\theta _2-\theta _1=-\pi/4$  and $arg(z _1z _2)=\pi/2$ (given)
$\implies \theta _2-\theta _1=-\pi/4$  and $\theta _2
+\theta _1=\pi/2$

$\implies 2\theta _2=\pi/4,\theta _2=\pi/8$
$\implies arg(z _2)=\pi/8$

The complex number $\dfrac{1 + 2i}{1 - i}$ lies in which quadrant of the complex plane.

  1. First

  2. Second

  3. Third

  4. Fourth


Correct Option: B
Explanation:
$\dfrac{1+2i}{1-i}$
$\Rightarrow \dfrac{1+2i}{1-i}\times \dfrac{1+i}{1+i}$
$=\dfrac{1+i+2i-2i^2}{1-i^2}=\dfrac{1+3i-2}{2}$
$=\dfrac{-1+3i}{2}$
$\therefore$ It lies in $2^{nd}$ Quadrant.

If $arg(z) < 0$, then $arg(-z)-arg(z)=$

  1. $\pi$

  2. $-\pi$

  3. $\dfrac{\pi}{2}$

  4. $-\dfrac{\pi}{2}$


Correct Option: A
Explanation:

Let $Z=re^{i\theta _1}$

$-Z=-re^{i\theta _1}$
$\implies -a\cos\theta _1-ib\sin\theta _1$
$\implies -a\cos(\pi+\theta _1)-ib\sin(\pi+\theta _1)$
$\implies re^{i(\pi+\theta _1)}$
$arg(-Z)-arg(Z)$
$\implies \pi+\theta _1-\theta _1\ \implies \pi$

Which of the given alternatives represent a point in Argand plane, equidistant from roots of the equation $(z+1)^4= 16z^4$?

  1. $(0,0)$

  2. $\left(-\dfrac{1}{3},0\right)$

  3. $\left(\dfrac{1}{3},0\right)$

  4. $\left(0,\dfrac{2}{\sqrt5}\right)$


Correct Option: C
Explanation:

Consider the given equation $(z+1)^4=16z^4$
$ \Rightarrow (z+1)^4=16z^4$

$ \Rightarrow z+1=(2^4z^4)^{\frac{1}{4}}$

$ \Rightarrow |z+1|=2|z|$

We know that $z=x+iy$

Therefore $|x+iy+1|=2|x+iy|$

$ \Rightarrow \sqrt{(x+1)^2+y^2}=2\sqrt{x^2+y^2}$

$ \Rightarrow (x+1)^2+y^2=4(x^2+y^2)$

$ \Rightarrow x^2+2x+1+y^2=4x^2+4y^2$

$ \Rightarrow 3x^2+3y^2-2x-1=0$

Divide throughout by 3 we get,
$ \Rightarrow x^2+y^2-\dfrac{2}{3}x-\dfrac{1}{3}=0$, which represents a circle.

We know that for the circle equation of the form $x^2+y^2+2gx+2hy+c=0$ the center of the circle is given by $(-g,-h)$

We have $3x^2+3y^2-2x-1=0$ where $g=-\dfrac{1}{3}, h=0$.

Hence the center is $(\dfrac{1}{3},0)$ which is equidistant from the root of the equation.

A particle starts from a point $z _0= I + i$, where $i
=\sqrt{-1}$ It moves horizontally away from origin by $2$ units and then
vertically away from origin by $3$ units to reach a point$ z _1$. From $z _1$
particle moves $\sqrt{5}$ units in the direction of $2\hat i + \hat j$ and
then it moves through an angle of $\cos e{c^{ - 1}}\sqrt 2 $ in anticlockwise
direction of a circle with centre at origin to reach a point $z _2$ . The arg $z _2$ is given by

  1. ${\sec ^{ - 1}}2$

  2. ${\cot ^{ - 1}}0$

  3. ${\sin ^{ - 1}}\left( {\dfrac{{\sqrt 3 - 1}}{{2\sqrt 2 }}} \right)$

  4. ${\cos ^{ - 1}}\left( {\dfrac{{ - 1}}{2}} \right)$


Correct Option: B

The number of solution of $z^2 + \bar{z} = 0$ is

  1. $5$

  2. $4$

  3. $2$

  4. $3$


Correct Option: B
Explanation:
Let $z=x+iy$.
Now,
$z^2+\overline{z}=0$
or, $x^2-y^2+2ixy+(x-iy)=0$
or, $(x^2-y^2+x)+i(2xy-y)=0$
Now comparing the real and imaginary part both sides we get,
$x^2-y^2+x=0$.....(1) and $2xy-y=0$.....(2).
From (2) we get, $x=\dfrac{1}{2}$ or $y=0$.
Now $x=\dfrac{1}{2}$ gives from (1) we get, $y=\pm \dfrac{\sqrt{3}}{2}$.
And $y=0$ gives from (1) we get, $x=0, 1$.
So the solution s are $(0,0), (1,0), \left(\dfrac{1}{2},\pm \dfrac{\sqrt{3}}{2}\right)$.
So we have $4$ solutions.

If $z \neq 0$, then $ \overset{100}{\underset{0}{\int}}arg(-|z|)dx =$

  1. $0$

  2. Not defined

  3. $100$

  4. $100\pi$


Correct Option: A

The complex no. $\dfrac{1+2i}{1-i}$ lies in which quadrant of the complex plane

  1. first

  2. second

  3. third

  4. fourth


Correct Option: B
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