Let $P (\frac{2 \sqrt{3}}{\sqrt{7}}, \frac{6}{\sqrt{7}}),$ Q, R and S be four points on the ellipse $9 x^{2} + 4 y^{2} = 36 .$ Let PQ and RS be mutually perpendicular and pass through the origin. If $\frac{1}{{(PQ)}^{2}} + \frac{1}{{(RS)}^{2}} = \frac{p}{q},$ where p and q are coprime, then p+q is equal to

see full answer

Your Exam Success, Personally Taken Care Of

1:1 expert mentors customize learning to your strength and weaknesses – so you score higher in school , IIT JEE and NEET entrance exams.

An Intiative by Sri Chaitanya

143

137

147

157

answer is D.

(Unlock A.I Detailed Solution for FREE)

Best Courses for You

JEE

NEET

Foundation JEE

Foundation NEET

CBSE

Detailed Solution

Given $r_{1} = \sqrt{\frac{12}{49} + \frac{36}{49}} = \frac{4 \sqrt{3}}{7}$
$\frac{{r_{1}}^{2} \cos^{2} θ}{4} + \frac{{r_{1}}^{2} \sin^{2} θ}{9} = 1 \Rightarrow \frac{\cos^{2} θ}{4} + \frac{\sin^{2} θ}{9} = \frac{1}{{r_{1}}^{2}}$

$\frac{{r_{2}}^{2} \sin^{2} θ}{4} + \frac{{r_{2}}^{2} \cos^{2} θ}{9} = 1 \Rightarrow \frac{\sin^{2} θ}{4} + \frac{\cos^{2} θ}{9} = \frac{1}{{r_{2}}^{2}}$
$∴ \frac{1}{{r_{1}}^{2}} + \frac{1}{{r_{2}}^{2}} = \frac{1}{4} + \frac{1}{9} = \frac{13}{36}$
$\frac{1}{{PQ}^{2}} + \frac{1}{{RS}^{2}} = \frac{1}{4 {r_{1}}^{2}} + \frac{1}{4 {r_{2}}^{2}} = \frac{1}{4} [\frac{13}{36}] = \frac{13}{144} = \frac{p}{q} ∴ p + q = 157$