1.

If P is a point on the line x + 4a = 0 and QR is the chord of contact of P with respect to y2 = 4ax, then ∠QOR (where O is the vertex) is equal to (A)  45°(B)  60°(C)  30°(D)  90°

Answer»

Correct option  (D)  90°

Explanation 

 Let P be (x1, y1) so that

x1 + 4a = 0 .....(1)

Chord of contact of P(x1,y1) is

yy1 - 2a(x + x1) = 0

⇒ yy1 - 2ax + 8a2 = 0

[∴ x1 = -4a  from Eq.(1)]

⇒ yy1 - 2a(x + x1) = 0

⇒ yy1 -2ax + 8a2 = 0

⇒ 2ax - y1y/8a2 = 1

Hence, the combined equation of the pair of lines OQ and OR is

y2 - 4ax(2ax - y1y/8a2)

In this equation, the coefficient of x2 + the coefficient of y2 = −1 + 1 = 0. Hence ∠QOR  = 90°



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