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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° |
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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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