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Pair of bisectors of angles - class-XI

Description: pair of bisectors of angles
Number of Questions: 31
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Tags: angles and triangles maths straight lines pair of straight lines two dimensional analytical geometry triangles
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If the line y = mx is one of the bisector of the lines $x^2 + 4xy - y^2 = 0$, then the value of no ___________.

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $\frac{\sqrt{5} - 1}{2}$

  2. $\frac{\sqrt{5} + 1}{2}$

  3. $-(\frac{\sqrt{5} + 1}{2})$

  4. $-(\frac{\sqrt{5} -1}{2})$

Show answer
Correct Option: A

The Straight lines represented by the equation $135{ x }^{ 2 }-136xy+33{ y }^{ 2 }=0$ are equally inclined to the line 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $x-2y=7$

  2. $x+2y=7$

  3. $x-2y=4$

  4. $3x+2y=4$

Show answer
Correct Option: B
Explanation:

Give pair of lines is $135{ x }^{ 2 }-136xy+33{ y }^{ 2 }=0$   ...(1)


The equation of bisector of angles between pair of lines (1) is

$\displaystyle \frac { { x }^{ 2 }-{ y }^{ 2 } }{ a-b } =\frac { xy }{ h } \Rightarrow \frac { { x }^{ 2 }-{ y }^{ 2 } }{ 135-33 } =\frac { xy }{ -68 } $

$\Rightarrow 2{ x }^{ 2 }+3xy-2{ y }^{ 2 }=0\Rightarrow \left( x+2y \right) \left( 2x-y \right) =0$

One of the bisectors is $x+2y=0$ which is parallel to the line $x+2y=7$.

Hence, the line $x+2y=7$ is equally inclined to the given lines.

If the bisectors of the lines $x^2 - 2pxy - y^2 = 0$ be $x^2 - 2qxy - y^2 = 0$. then

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. 2p + q = 0

  2. 2p + 3q = 0

  3. pq = 1

  4. pq + 1 = 0

Show answer
Correct Option: A

If the pair of straight lines $x^{2}-2pxy-y^{2}= 0$ and $x^{2}-2qxy-y^{2}= 0$ be such that each pair bisects the angle between the other pair, then

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $p= -q$

  2. $pq= 1$

  3. $pq= -1$

  4. $p= q$

Show answer
Correct Option: C
Explanation:

Given equations are $\displaystyle  x^{2}-2qxy-y^{2}=0 ...(1) $ $\displaystyle  x^{2}-2pxy-y^{2}=0 ...(2) $ Joint equation of angle bisector of the line (i) and (ii) are same $\displaystyle \therefore qx^{2}+2xy-qy^{2}=0....(3) $. 


Now (2).and (3) are same, taking the ratio of their coefficients


$\displaystyle \therefore \frac{1}{q}=\frac{-p}{1}\Rightarrow pq=-1$

2x + y - 4 = 0 is a besector of angles between the lines a(x - 1) + b(y - 2) = 0, c(x - 1) + d(y - 2) = 0 the other angular bisector is _______________.

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. x - 2y + 1 = 0

  2. x - 2y - 3 = 0

  3. x - 2y + 3 = 0

  4. x + 2y - 5 = 0

Show answer
Correct Option: C
Explanation:
We have $a\left(x-1\right)+b\left(y-2\right)=0$       .....$(1)$
and $c\left(x-1\right)+d\left(y-2\right)=0$       .....$(2)$

Clearly $\left(1,2\right)$ lie on both the lines and hence $\left(1,2\right)$ is their point of intersection.

Both the bisectors will pass through $\left(1,2\right)$
One of the bisector is $2x+y-4=0$

Other bisector will be perpendicular to this bisector.
Hence its equation will be $x-2y=\lambda$

It passes through $\left(1,2\right)$
$\Rightarrow\,1-4=\lambda$
$\Rightarrow\,\lambda=-3$

Hence the equation is $x-2y=-3$ or $x-2y+3=0$

The equations of the bisectors of that angle between the lines $x+2y-11=0,:3x+6y-5=0$ which contains the point $\left(1,-3\right)$ is 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $3x=19$

  2. $3y=7$

  3. $3x=19$ and $3y=7$

  4. None of these

Show answer
Correct Option: A

The line $L$ has intercepts $a$ and $b$ on the co-ordinate axes keeping the origin fixed, the co-ordinate axes are related through a fixed angle. If the same line has intercepts c and d then

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $ \displaystyle \frac{1}{a^{2}}+\frac{1}{c^{2}}= \frac{1}{b^{2}+d^{2}} $

  2. $ \displaystyle \frac{1}{a^{2}}+\frac{1}{b^{2}}= \frac{1}{c^{2}}+\frac{1}{d^{2}} $

  3. $ \displaystyle a^{2}+c^{2}= b^{2}+d^{2} $

  4. $ \displaystyle a^{2}+b^{2}= c^{2}+d^{2} $

Show answer
Correct Option: B
Explanation:

Suppose we state he coordinate axis in the anti-clockwise direction through an angle $\alpha$.
The equation of the line $\alpha$ with respect to old axes is $\displaystyle\frac{x}{a}+\frac{y}{b}=1$
In this equation replacing $x$ by $x\cos\alpha-y\sin\alpha$
The equation of the line with respect to new axes is
$\displaystyle\frac{x\cos\alpha-y\sin\alpha}{a}+\frac{x\sin\alpha+y\cos\alpha}{b}=1$
$\displaystyle\Rightarrow x\left( \frac { \cos { \alpha  }  }{ a } +\frac { \sin { \alpha  }  }{ b }  \right) +y\left( \frac { \cos { \alpha  }  }{ b } -\frac { \sin { \alpha  }  }{ a }  \right) =1$   ...(1)
The intercept mode by (1) on the co-ordinate axes are given as $c$ and $d$.
Therefore, $\displaystyle\frac{1}{c}=\frac { \cos { \alpha  }  }{ a } +\frac { \sin { \alpha  }  }{ b } $ and $\displaystyle\frac{1}{d}=\frac { \cos { \alpha  }  }{ b } -\frac { \sin { \alpha  }  }{ a }$
Squaring and adding, we get $ \displaystyle \frac{1}{c^{2}}+\frac{1}{d^{2}}=\frac{1}{a^{2}}+\frac{1}{b^{2}} $

$P: x^{2}-y^{2}+2y-1=0$
$L: x+y=3$

Equation of the angle bisectors of the pairs of lines P is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $xy-y=0$

  2. $xy-x=0$

  3. $xy=0$

  4. $xy+y=0$

Show answer
Correct Option: B
Explanation:

$P:{ x }^{ 2 }-{ \left( y-1 \right)  }^{ 2 }=0$

$\Rightarrow x+y-1=0$ and $x-y+1=0$

Equation of the angle bisector is

$\cfrac { A _1x+B _1y+C _1 }{ \sqrt {A _1^2+B _1^2 }  } =\pm \cfrac { A _2x+B _2y+C _2 }{ \sqrt {A _2^2+B _2^2}  } $
$\cfrac { x+y-1 }{ \sqrt { 2 }  } =\pm \cfrac { x-y+1 }{ \sqrt { 2 }  } $

$\Rightarrow x=0$ or $y-1=0$

If pairs of lines $3x^{2}-2pxy-3y^{2}=0$ and $5x^{2}-2qxy-5y^{2}=0$ are such that each pair bisects the angle between the other pair, then $pq$ is equal to

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $-1$

  2. $-3$

  3. $-5$

  4. $-15$

Show answer
Correct Option: D
Explanation:
Given pairs 
$3x^2-2pxy-3y^2=0$----(1)

$5x^2-2qxy-5y^2=0$----(2)

Equation (1) can be written as 
$3(x^2-y^2)=2pxy$----(3)

Eq of angle bisector of eq (2)
$\dfrac{x^2-y^2}{5+5}=\dfrac{xy}{-q}$

$\dfrac{x^2-y^2}{10}=\dfrac{xy}{-q}$----(4)

Now as the given in question the eq of angle bisector of one pair bisects the other pair

So dividing eq (4) by (3)
$\dfrac{\dfrac{x^2-y^2}{10}}{3(x^2-y^2)}=\dfrac{\dfrac{xy}{-q}}{2pxy}$

$\dfrac{1}{30}=\dfrac{1}{-2pq}$

$pq=-15$ 

Slope of a bisector of the angle between the lines $4x^{2}-16xy-7y^{2}=0$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $\displaystyle \frac{11+\sqrt{377}}{16}$

  2. $\displaystyle \frac{11-\sqrt{377}}{16}$

  3. $\displaystyle \frac{-3+2\sqrt{3}}{7}$

  4. $\displaystyle \frac{-3-2\sqrt{3}}{7}$

Show answer
Correct Option: A,B
Explanation:
Given pair 
$4x^2-16xy-7y^2=0$
On comparing given eq with $ax^2+2hxy+by^2=0$
$a=4,b=-7,h=-8$
Eq of pair of Angle bisector 
$\dfrac{x^2-y^2}{a-b}=\dfrac{xy}{h}$
$\dfrac{x^2-y^2}{11}=\dfrac{xy}{-8}$
$-8x^2+8y^2=11xy$
$8y^2-11xy-8x^2=0$
$y=\dfrac{-(-11x)\pm\sqrt{121x^2+256x^2}}{16}$

$y=\dfrac{11x\pm\sqrt{377x^2}}{16}$

$y=\dfrac{11x\pm\sqrt{377}x}{16}$

$y=\left (\dfrac{11\pm\sqrt{377}}{16}  \right )x$
Comparing above eq with $y=mx+c$
$m=\left (\dfrac{11+\sqrt{377}}{16}  \right )$ and $\left (\dfrac{11-\sqrt{377}}{16}  \right )$

$P: 2x^{2}-axy+6y^{2}=0$
$Q: 3x^{2}-8xy+4y^{2}=0$
If the bisectors of the angles between the lines represented by $P$ and $Q$ are same, the value of $a$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $8$

  2. $\dfrac{16}{3}$

  3. $32$

  4. $-16$

Show answer
Correct Option: C
Explanation:

Equation of angle bisector of $2x^{ 2 }-axy+6y^{ 2 }=0$ is

$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 2-6 } =\cfrac { xy }{ -a/2 } \Rightarrow a{ x }^{ 2 }-a{ y }^{ 2 }-8xy=0$   ----------(1)

And equation of angle bisector of $3x^{ 2 }-8xy+4y^{ 2 }=0$ is

$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 3-4 } =\cfrac { xy }{ -4 } \Rightarrow 4{ x }^{ 2 }-4{ y }^{ 2 }-xy=0$  -----------(2)

From (1) and (2), we get $a=32$

If the lines represented by $x^2-2pxy-y^2=0$ are rotated about the origin through an angle $\theta,$ one in clockwise direction and other in anti-clockwise direction, then the equation of the bisector of the angle between the lines in the new positions is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $px^2+2xy-py^2=0$

  2. $px^2+2xy+py^2=0$

  3. $x^2-2pxy-y^2=0$

  4. None of these

Show answer
Correct Option: A
Explanation:
Given eq 
$x^2-2pxy-y^2=0$
comparing above eq with general form of eq $ax^2+2hxy+by^2=0$
$a=1,h=-p,b=-1$
Now the line is rotated one in clockwise and other is anticlockwise so the both eq are replaced by each other and form the same eq as it was so here we finding eq of angle bisector by formula
$\dfrac{x^2-y^2}{a-b}=\dfrac{xy}{h}$
$\dfrac{x^2-y^2}{1+1}=\dfrac{xy}{-p}$
$\dfrac{x^2-y^2}{2}=\dfrac{xy}{-p}$
$(-p)(x^2-y^2)=2xy$
$-px^2+py^2=2xy$
$px^2+2xy-py^2=0$
 
$\displaystyle ax^{2}+2hxy+by^{2}=0$ represents a pair of straight lines through origin & angle between them is given by
$\displaystyle \tan \theta=\frac{2\sqrt{h^{2}-ab}}{a+b}$. If the lines are perpendicular then $\displaystyle a+b=0 $ and the equation of bisectors is given by  $\displaystyle \frac{x^{2}-y^{2}}{a-b}=\frac{xy}{h}$
The general equation of second degree given by
$\displaystyle ax^{2}+2hxy+by^{2}+2gx+2fy+c=0$ represent a pair of straight lines if $\displaystyle \triangle =0 $ or 
$ \displaystyle \begin{vmatrix}a&h  &g \\ h&b  &f \\ g&f  &c \end{vmatrix}=0 $ or $\displaystyle abc+2fgh-af^{2}-bg^{2}-ch^{2}=0$
On the basis of above information answer the following question

The Joint equation of the bisectors of the angle between the lines represented by $\displaystyle ax^{2}+2hxy+by^{2}=0 $ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. a pair of perpendicular lines

  2. a pair of parallel lines

  3. a pair of intersecting lines but not $\displaystyle \perp$er

  4. None of these

Show answer
Correct Option: A
Explanation:

Equation of bisectors of angle between the pair of straight line $\displaystyle ax^{2}+2hxy+by^{2}=0$ is given by $\displaystyle \frac{x^{2}-y^{2}}{a-b}=\frac{xy}{h}$


$\displaystyle\Rightarrow hx^{2}-\left ( a-b \right )xy-hy^{2}=0$

Here coefficient of $x^{2}+$ coefficient of $y^{2}=h+(-h)=0$

$\displaystyle \therefore $ above equation represent the pair of $\perp$er lines

Hence choice (a) is correct.

The equation $a^2 x^2 + 2h(a+b) xy + b^2 y^2 = 0$ and $ax^2 + 2hxy + by^2 = 0$ represent

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. two pairs of perpendicular straight lines

  2. two pairs of parallel straight lines

  3. two pairs of straight lines which are equally inclined to each other

  4. None of these

Show answer
Correct Option: C
Explanation:

$ax^2+2hxy+by^2=0$
Equation of the angle bisectors is given by
$\dfrac{x^2-y^2}{a-b}=\dfrac{xy}{h}$ ...(i)

For $ a^2x^2+2h(a+b)xy+b^2y^2=0$
The equations of the angle bisector is given by
$\dfrac{x^2-y^2}{a^2-b^2}=\dfrac{xy}{h(a+b)}$
 $\dfrac{x^2-y^2}{a-b}=\dfrac{xy}{h}$ ...(ii)
Since (i) is equal to (ii), the above lines are equally inclined to each other.

If one of the lines of $my^2 + (1-m^2) xy - mx^2 = 0$ is a bisector of the angle between the lines $xy = 0$, then $m$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $3$

  2. $2$

  3. $-\dfrac{1}{2}$

  4. $-1$

Show answer
Correct Option: D
Explanation:

$my^2+(1-m^2)xy-mx^2$
$(my+x)(y-mx)=0$
Therefore the lines are $y=mx$ and $y=\dfrac{-x}{m}$ ...(i)
The angle bisectors of the $xy=0$ is $y=x$ and $y=-x$
Comparing the slopes with the equations of the (i) we get
$m=\pm1$

If one of the lines of is $my^{2}+\left ( 1-m^{2} \right )xy-mx^{2}=0$ is a bisector of the angle between the lines $\displaystyle xy = 0,$ then $m$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $\displaystyle1$

  2. $\displaystyle2$

  3. $\displaystyle-\frac{1}{2}$

  4. $\displaystyle-1$

Show answer
Correct Option: A,D
Explanation:

Given pair of lines is
$\displaystyle { my }^{ 2 }+\left( 1-{ m }^{ 2 } \right) xy-{ mx }^{ 2 }=0\Rightarrow m{ \left( \frac { y }{ x }  \right)  }^{ 2 }+{ \left( 1-m \right)  }^{ 2 }\frac { y }{ x }- m=0$    ...(1)

Lines $xy=0$ are $x=0$ and $y=0.$
i.e the axes bisector of angle between the axes are $y=x$ and $y=-x$ $\displaystyle \Rightarrow \frac { y }{ x } =1$ or $\displaystyle \frac { y }{ x } =-1$

If $\displaystyle \frac { y }{ x } =1$ is represented by (1), then
$m{ \left( 1 \right)  }^{ 2 }+\left( 1-{ m }^{ 2 } \right) \left( -1 \right) -m=0\Rightarrow 1-{ m }^{ 2 }=0\Rightarrow m=\pm 1$

If $\displaystyle \frac { y }{ x } =-1$ is represented by (1), then
$m{ \left( -1 \right)  }^{ 2 }+\left( 1-{ m }^{ 2 } \right) \left( -1 \right) -m=0\Rightarrow 1-{ m }^{ 2 }=0\Rightarrow m=\pm 1$
In both cases either $m=1$ or $m=-1.$

If the pair of straight lines ${x^2} - 2pxy - {y^2} = 0$ and ${x^2} - 2qxy - {y^2} = 0$ be such that each pair bisects the angle between the other pair,then:

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $pq=-1$

  2. $p=q$

  3. $p=-q$

  4. $pq=1$

Show answer
Correct Option: B

The pairs of straight lines $ax^{2}+2hxy-ay^{2}=0$ and $hx^{2}-2axy-hy^{2}=0$ are such that

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. one pair bisects the angles between the other pair

  2. the lines of one pair are equally inclined to the lines of the other pair

  3. the lines of one pair are perpendicular to the `lines of the other pair

  4. none of these

Show answer
Correct Option: A,B
Explanation:

Given pairs of straight lines are $ax^{2}+2hxy-ay^{2}=0$ and $hx^{2}-2axy-hy^{2}=0$.

Pair of angular bisectors of $ax^{2}+2hxy-ay^{2}=0$ is $h(x^2-y^2)=(a-(-a))xy$

$\Rightarrow hx^2-2axy-hy^2$

$\therefore$ One pair bisects the angle between the other.

Clearly, if one pair bisects the angle between the other, the lines of one pair are equally inclined to the lines of the other pair.
Hence, option A and B.

If one of the lines of $my^2 + (1- m^2) xy - mx^2 = 0$ is a bisector of the angle between the lines $xy = 0$, then $m$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $1$

  2. $2$

  3. $\displaystyle \frac{-1}{2}$

  4. $-1$

Show answer
Correct Option: A,D
Explanation:

Angle bisectors of $xy=0$ are $x+y=0 $  and $x=y$
$my^2+xy-m^2 xy -mx^2=0$
$\therefore y(my+x)-mx(my+x)=0$
$\therefore (y-mx)(my+x)=0$
Comparing $y-mx=0  $ and $  my+x=0  $  with  $y=x $ and  $ y=-x$,  we get $m=\pm 1$ 

The straight lines $7x^{2}+6xy+4y^{2}=0$ have the same pair of bisectors as those of the lines given by

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $49x^{2}+66xy+16y^{2}=0$

  2. $10x^{2}+6xy+7y^{2}=0$

  3. $5x^{2}+6xy+2y^{2}=0$

  4. $4x^{2}-6xy+7y^{2}=0$

Show answer
Correct Option: A,B,C,D
Explanation:

For $7x^{ 2 }+6xy+4y^{ 2 }=0$ 
Equation of angle bisector is 
$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 7-4 } =\cfrac { xy }{ 3 } \Rightarrow { x }^{ 2 }-xy-{ y }^{ 2 }=0$
For Option A 
Equation of angle bisector of $49x^{ 2 }+66xy+16y^{ 2 }=0$ is
$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 33 } =\cfrac { xy }{ 33 } \Rightarrow { x }^{ 2 }-xy-{ y }^{ 2 }=0$
For Option B
Equation of angle bisector of $10x^{ 2 }+6xy+7y^{ 2 }=0$ is
$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 3 } =\cfrac { xy }{ 3 } \Rightarrow { x }^{ 2 }-xy-{ y }^{ 2 }=0$
For Option C
Equation of angle bisector of $5x^{ 2 }+6xy+2y^{ 2 }=0$ is
$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ 3 } =\cfrac { xy }{ 3 } \Rightarrow { x }^{ 2 }-xy-{ y }^{ 2 }=0$
For Option D
Equation of angle bisector of $4x^{ 2 }-6xy+7y^{ 2 }=0$ is
$\cfrac { { x }^{ 2 }-{ y }^{ 2 } }{ -3 } =\cfrac { xy }{ -3 } \Rightarrow { x }^{ 2 }-xy-{ y }^{ 2 }=0$

$\displaystyle ax^{2}+2hxy+by^{2}=0$ represents a pair of straight lines through origin & angle between them is given by
$\displaystyle \tan \theta=\frac{2\sqrt{h^{2}-ab}}{a+b}$. If the lines are perpendicular then $\displaystyle a+b=0 $ and the equation of bisectors is given by  $\displaystyle \frac{x^{2}-y^{2}}{a-b}=\frac{xy}{h}$
The general equation of second degree given by
$\displaystyle ax^{2}+2hxy+by^{2}+2gx+2fy+c=0$ represent a pair of straight lines if $\displaystyle \triangle =0 $ or 
$ \displaystyle \begin{vmatrix}a&h  &g \\ h&b  &f \\ g&f  &c \end{vmatrix}=0 $ or $\displaystyle abc+2fgh-af^{2}-bg^{2}-ch^{2}=0$
On the basis of above information answer the following question

Let $\displaystyle  f _{1}\left (x,y  \right )=ax^{2}+2hxy+by^{2}=0$ and let $\displaystyle  f _{i+1}\left (x,y  \right )=0 $ denotes the equation of bisectors of $\displaystyle  f _{i}\left (x,y  \right )=0 \forall $ $ i=1,2,3 $ then equation of $\displaystyle  f _{3}\left (x,y  \right )=0$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $\displaystyle \left (a-b \right )x^{2}-4hxy+\left (a-b \right )y^{2}=0 $

  2. $\displaystyle \left (a-b \right )x^{2}-4hxy-\left (a-b \right )y^{2}=0 $

  3. $\displaystyle \left (a-b \right )x^{2}+4hxy-\left (a-b \right )y^{2}=0 $

  4. $\displaystyle \left (a-b \right )x^{2}+4hxy+\left (a-b \right )y^{2}=0 $

Show answer
Correct Option: C

The sum and product of the slopes of a pair of straight lines are the arithmetic and the geometric means of 9 and 16 respectively. The equation of the bisectors of the angles between the lines through the origin are 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $24x^{2}-25xy+2y^{2}=0$

  2. $25x^{2}+44xy-25y^{2}=0$

  3. $11x^{2}-25xy-11y^{2}=0$

  4. none of these

Show answer
Correct Option: B
Explanation:

Let the equation of lines passing through origin be $y-m _1x=0,y-m _2x=0$
$\therefore$The combined equation of pair of straight lines $=(y-m _1x)(y-m _2x)=0$

$\Rightarrow m _1m _2x^2-(m _1+m _2)xy+y^2=0$

But given sum of the slopes ,$m _1+m _2=\frac{(9+16)}{2}=\frac{25}{2}$
product of the slopes ,$m _1m _2=\sqrt(9*16)=12$
On subtituting these values in the above equation.
$\Rightarrow 12x^2-\displaystyle\frac{25}{2}xy+y^2=0$

$\Rightarrow 24x^2-25xy+2y^2=0$ comparing with general equation of pair of straight lines passing through origin $ax^2+2hxy+by^2=0$
$\Rightarrow a=224,h=\displaystyle\frac{-25}{2},b=2$
If $ax^2+2hxy+cy^2=0$ is pair of equation of line passing through origin then pair of equation of the angular bisector of these pair of lines is obtained by
 $h(x^2-y^2)=(a-b)xy$

$\therefore$ The required pair of equation of angular bisector is $h(x^2-y^2)=(a-b)xy$
$\Rightarrow \displaystyle\frac{-25}{2}(x^2-y^2)=(24-2)xy$
$\Rightarrow \displaystyle\frac{25}{2}*(x^2-y^2)=-22xy$

$\Rightarrow (25x^2-25y^2)=-44xy$

$\Rightarrow 25x^2+44xy-25y^2=0$

If $\displaystyle y=mx$ bisects the angle between the lines $\displaystyle x^{2}\left ( \tan ^{2}\theta +\cos ^{2}\theta  \right )+2xy\tan \theta -y^{2}\sin ^{2}\theta =0$  when $\displaystyle \theta =\dfrac\pi3$ the value of $m$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $\displaystyle \frac{-2- \sqrt 7}{ \sqrt 3}$

  2. $\displaystyle \frac{ \sqrt 7-2}{ \sqrt 3}$

  3. $\displaystyle 2 \sqrt 7 $

  4. $\displaystyle 2 \sqrt 3 $

Show answer
Correct Option: A,B
Explanation:
Equation of the bisectors of the angles between the given lines is

$\displaystyle \dfrac{x _2-y _2}{a-b}=\dfrac{xy}{h }$

Equation of the bisectors of the angles between the given lines is
$\displaystyle \frac{x^{2}-y^{2}}{\tan ^{2}\theta +\cos ^{2}\theta +\sin ^{2}\theta }=\frac{xy}{\tan \theta }$

$\displaystyle \Rightarrow \frac{x^{2}-y^{2}}{1+\tan ^{2}\theta  }=\frac{xy}{\tan \theta }$

$\displaystyle \Rightarrow \frac{x^{2}-y^{2}}{1+3  }=\frac{xy}{\sqrt 3 } \ when \ \ \theta=\pi/3$

Which satisfied by $y=mx $ if

$\displaystyle \frac{1-m^{2}}{4}=\frac{m}{\sqrt 3}$

$\displaystyle \Rightarrow \sqrt 3 m^{2}+4m-\sqrt 3=0$

$\displaystyle \Rightarrow m=\frac{-2\pm \sqrt 7}{\sqrt 3}$

If two of the lines represented by $ x^{4} + x^{3} y + cx^{2}y^{2} -xy^{3} + y^{4} =0$ bisect the angle between the other two, then the value of $c$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $0$

  2. $-1$

  3. $1$

  4. $-6$

Show answer
Correct Option: D
Explanation:

Since the product of the slopes of the four lines represented by the given equation is $1$ and a pair of lines represent the bisectors of the angles between the other two, the product of the slopes of each pair is $-1$. So let the equation of one pair be $ax^{2} + 2hxy -ay^{2} = 0$

The equation of its bisectors is $ \displaystyle \frac{x^{2}-y^{2}}{2a}=\frac{xy}{h} $

By hypothesis $ x^{4} +x^{3}y + cx^{2} y^{2}-xy^{3} + y^{4} $ $= (ax^{2} + 2hxy -ay^{2}) (hx^{2} -2axy -hy^{2})$ 

$ = ah(x^{4} + y^{4}) + 2(h^{2} -a^{ 2}) (x^{3}y- xy^{3}) -6ahx^{2}y^{2} $ 

Comparing the respective coefficients we get

$ah = 1 $ and $c = -6ah = -6$

The line $y=3x$ bisects the angle between the lines $ax^{2}+2axy+y^{2}=0$ if ${a}=$ 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $3$

  2. $11$

  3. $\displaystyle \frac{3}{11}$

  4. $\displaystyle \frac{11}{3}$

Show answer
Correct Option: C
Explanation:

Pair of angle bisectors represented by
$ax^2+2hxy+by^2=0$   is given by
$\dfrac{x^2-y^2}{a-b}=\dfrac{xy}{h}$
$\therefore ax^2+2axy+y^2=0$
pair of angle bisector is,
$\dfrac{x^2-y^2}{a-1}=\dfrac{xy}{a}$
$ax^2-ay^2=(a-1)xy$
Given  $ y=3x$  is one of angle bisector of given lines
$m=\dfrac{y}{x},$        $am^2+(a-1)x-a=0$
$m=3$ is satisfied to this equation
$9a+3a-3-a=0$
$11a=3$
$\therefore a=\dfrac{3}{11}$

The angle of intersection of the curves  $x ^ { 2 } + 4 y ^ { 2 } = 32$  and  $x ^ { 2 } - y ^ { 2 } = 12$  at any point of their intersection is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

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

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

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

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

Show answer
Correct Option: A
Explanation:
${ x }^{ 2 }+{ 4y }^{ 2 }=32\quad \longrightarrow \left( i \right) $
${ x }^{ 2 }-{ y }^{ 2 }=12\quad \longrightarrow \left( ii \right) $
Solving, $y=\pm 2$
              $x=\pm 4$
$\therefore$   Point of ${ X }^{ n }$ are $\left( 4,2 \right) ,\left( 4,-2 \right) ,\left( -4,2 \right) ,\left( -4,-2 \right) $
At $(4,2)$
${ m } _{ 1 }=\dfrac { -x }{ 4y } $    [differentiating $(i)$ wrt $x$]
$=\dfrac { 2 }{ -16 } =\dfrac { -1 }{ 8 } $
${ m } _{ 2 }=y/x$    [differentiating $(ii)$ wrt $x$]
$=\dfrac { 2 }{ 4 } =\dfrac { 1 }{ 2 } $
$\tan\theta =\dfrac { \left| { m } _{ 1 }-{ m } _{ 2 } \right|  }{ 1+{ m } _{ 1 }{ m } _{ 2 } } =\dfrac { \left| \dfrac { -1 }{ 8 } -\dfrac { 1 }{ 2 }  \right|  }{ 1-\dfrac { 1 }{ 16 }  } =\dfrac { \dfrac { 2+8 }{ 16 }  }{ \dfrac { 15 }{ 16 }  } =\dfrac { 10 }{ 15 } =\dfrac { 2 }{ 3 } $
$\Rightarrow \theta ={ \tan }^{ -1 }\left( 2/3 \right) \simeq \pi /6$              [A]

Family of lines represented by the equation $(\cos \theta)x+(\cos \theta -\sin \theta)y-3(3\cos \theta+\sin \theta)=0$ passes through a fixed point $M$ for all real value of $\theta$. Find $M$ 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $(6,3)$

  2. $(3,6)$

  3. $(-6,2)$

  4. $(3,-6)$

Show answer
Correct Option: A
Explanation:

Let us consider the problem:

$\left( {\left( {\cos \theta  + \sin \theta } \right)x + \cos \theta  - \sin \theta } \right)y - 3\left( {3\cos \theta  + \sin \theta } \right) = 0$
$ \Rightarrow \cos \theta \left( {x + y - 9} \right) + \sin \theta \left( {x - y - 3} \right) = 0$
$ \Rightarrow $ $\left( {x + y - 9} \right) + \tan \theta \left( {x - y - 3} \right) = 0$
${L _1} + K{L _2} = 0$(pass through intersection of ${L _1}$ and ${L _2}$ for all value of $K$)
$x+y-9=0$
$ \Rightarrow $ $x - y - 3 = 0$ 
Hence,
$x+y=9$
$x-y=3$
hence the intersection point is $(6,3)$

If the equation $a{x}^{2}+2hxy+b{y}^{2}=0$ represents a pair of lines then  the equation of the pair of lines of angular bisectors is $h({x}^{2}-{y}^{2})-(a-b)xy=0$

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. True

  2. False

Show answer
Correct Option: A

If the line $y = mx$ bisects the angle between the line $ax^2 + 2h\ xy + by^2 = 0$ then $m$ is a root of the quadratic equation :

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $hx^2 + (a - b)x - h = 0$

  2. $x^2 +h(a - b)x - 1 = 0$

  3. $(a - b)x^2 + hx - (a - b) = 0$

  4. $(a - b)x^2 - hx - (a - b) = 0$

Show answer
Correct Option: A
Explanation:

Equation of bisectors of the pair of straight lines $ax^2+2hxy+by^2=0$ is

$h(x^2-y^2)-(a-b)xy=0$......(1).

Since $y=mx $ is given to be the bisector of the pair of straight lines, then the line will satisfy the equation (1).

Then we get,
$h(1-m^2)-(a-b)m=0$

$hm^2+(a-b)m-h=0$.

So $m$ satisfies the equation $hx^2+(a-b)x-h=0$.

Joint equation of perpendicular lines passing through $(0,0)$ one of which is parallel to $6x-4y+3=0$ is

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. $6x^{2}-5xy-6y^{2}=0$

  2. $6x^{2}+5xy-6y^{2}=0$

  3. $5x^{2}+5xy-6y^{2}=0$

  4. $6x^{2}-5xy-5y^{2}=0$

Show answer
Correct Option: A

The equation of the bisector of the obtuse angle between the lines 3x-4y+7=0 and 12x+5y-2=0 is: 

maths pair of straight lines bisection of angle pair of bisectors of angles bisector of angle between lines

  1. 21 x+77y-101=0

  2. 21 x+77 y+101=0

  3. 21x-77y-101=0

  4. 21x-77y+101=0

Show answer
Correct Option: A
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