JEE Coordinate (7)!

Geometry Level 5

A line passes through the point P = ( 2 , 3 ) P=(2,3) and makes an angle θ \theta ( 0 θ < π ) (0 \le \theta < \pi) with positive direction of x x -axis. If it meets the lines represented by x 2 2 x y y 2 = 0 x^2-2xy-y^2 = 0 at points Q Q and R R , and P Q P R = 17 PQ \cdot PR =17 , then find the number of possible values of θ \theta .


This is a part of my set Practice for JEE 2017.

2 1 3 0 Infinitely many 4 6 5

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1 solution

Aryaman Maithani
Jun 22, 2018

Any point on the line making an angle θ \theta with the x-axis and passing through the point ( 2 , 3 ) (2, 3) can be represented by ( 2 + r cos ( θ ) , 3 + r sin ( θ ) ) (2+r\cos(\theta), 3+r\sin(\theta)) where r |r| is the distance of the point from ( 2 , 3 ) (2, 3) .

Since we are interested in the intersection of this line with the given pair of straight lines, the above point satsfies the equation of the pair of straight lines:

( 2 + r cos ( θ ) ) 2 2 ( 2 + r cos ( θ ) ) ( 3 + r sin ( θ ) ) ( 3 + r sin ( θ ) ) 2 = 0 (2+r\cos(\theta))^2 - 2(2+r\cos(\theta))(3+r\sin(\theta)) - (3+r\sin(\theta))^2 = 0

r 2 ( sin θ cos θ ) 2 + 2 r ( sin θ cos θ ) 17 = 0 r^2 (\sin\theta - \cos\theta)^2 + 2r(\sin\theta - \cos\theta) - 17 = 0

Since the moduli of the roots of the above equation are the distances of the intersection points from ( 2 , 3 ) (2, 3) :

17 ( sin θ cos θ ) 2 = 17 \Bigg|\dfrac{-17}{(\sin\theta - \cos\theta)^2}\Bigg| = 17

( sin θ cos θ ) 2 = 1 \implies(\sin\theta - \cos\theta)^2=1

1 2 sin θ cos θ = 1 \implies 1 - 2\sin\theta\cos\theta = 1

s i n θ cos θ = 0 \implies sin\theta\cos\theta = 0

θ = 0 , π 2 \theta = \boxed{0, \dfrac{\pi}{2}}

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