If the normals at two points P and Q of a parabola y2=4ax intersect at R on the curve, then the product of ordinates of P and Q is
A
4a2
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B
2a2
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C
−4a2
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D
8a2
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Solution
The correct option is D8a2 If the normal at two points P(t1) and Q(t2) of a parabola y2=4ax intersect at R on the curve, then ⇒t3=−t1−2t1 Also, t3=−t2−2t2 ⇒−t1−2t1=−t2−2t2 ⇒t2−t1=2(1t1−1t2) t1t2=2 ∴ Product of ordinates =2at1×2at2=8a2