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Questions  

If the normal at the point P(θ) to the ellipse x214+y25=1

intersect it again at the point Q(2θ), then cos θ=

a
-23
b
23
c
-67
d
67

detailed solution

Correct option is A

Normal at P(14cos⁡ θ,5sin⁡ θ) to the ellipse is14xcos⁡ θ−5ysin⁡ θ=14−5=9Since it passes through Q(2θ)14×14cos⁡ 2θcos⁡ θ−5×5sin⁡ 2θsin⁡ θ=9⇒142cos2⁡ θ−1cos⁡ θ−5×2sin⁡ θcos⁡ θsin⁡ θ=9⇒18cos2⁡ θ−9cos⁡ θ−14=0⇒(3cos⁡ θ+2)(6cos⁡θ−7)=0⇒cos⁡ θ=−2/3.

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