Question

The combined equation of the lines $${l_1},\,{l_2}$$  is $$2{x^2} + 6xy + {y^2} = 0$$     and that of the lines $${m_1},\,{m_2}$$  is $$4{x^2} + 18xy + {y^2} = 0.$$     If the angle between $${l_1}$$ and $${m_2}$$ be $$\alpha $$ then the angle between $${l_2}$$ and $${m_1}$$ will be :

A. $$\frac{\pi }{2} - \alpha $$
B. $$2\alpha $$
C. $$\frac{\pi }{4} + \alpha $$
D. $$\alpha $$  
Answer :   $$\alpha $$
Solution :
The combined equation of bisectors of angles between the lines of the first pair is $$\frac{{{x^2} - {y^2}}}{{2 - 1}} = \frac{{xy}}{3},$$    and that of the other pair is $$\frac{{{x^2} - {y^2}}}{{4 - 1}} = \frac{{xy}}{9}$$
As these equations are the same, the two pairs are equally inclined to each other.

Releted MCQ Question on
Geometry >> Straight Lines

Releted Question 1

The points $$\left( { - a, - b} \right),\left( {0,\,0} \right),\left( {a,\,b} \right)$$     and $$\left( {{a^2},\,ab} \right)$$  are :

A. Collinear
B. Vertices of a parallelogram
C. Vertices of a rectangle
D. None of these
Releted Question 2

The point (4, 1) undergoes the following three transformations successively.
(i) Reflection about the line $$y =x.$$
(ii) Translation through a distance 2 units along the positive direction of $$x$$-axis.
(iii) Rotation through an angle $$\frac{p}{4}$$ about the origin in the counter clockwise direction.
Then the final position of the point is given by the coordinates.

A. $$\left( {\frac{1}{{\sqrt 2 }},\,\frac{7}{{\sqrt 2 }}} \right)$$
B. $$\left( { - \sqrt 2 ,\,7\sqrt 2 } \right)$$
C. $$\left( { - \frac{1}{{\sqrt 2 }},\,\frac{7}{{\sqrt 2 }}} \right)$$
D. $$\left( {\sqrt 2 ,\,7\sqrt 2 } \right)$$
Releted Question 3

The straight lines $$x + y= 0, \,3x + y-4=0,\,x+ 3y-4=0$$         form a triangle which is-

A. isosceles
B. equilateral
C. right angled
D. none of these
Releted Question 4

If $$P = \left( {1,\,0} \right),\,Q = \left( { - 1,\,0} \right)$$     and $$R = \left( {2,\,0} \right)$$  are three given points, then locus of the point $$S$$ satisfying the relation $$S{Q^2} + S{R^2} = 2S{P^2},$$    is-

A. a straight line parallel to $$x$$-axis
B. a circle passing through the origin
C. a circle with the centre at the origin
D. a straight line parallel to $$y$$-axis

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Straight Lines


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