Let x=9 be a directrix of an ellipse E, whose centre is at the origin and eccentricity is 13. Let P( , 0), >0, be a focus of E and AB be a chord passing through P. Then the locus of the mid point of AB is :
- 9y2=8x(1-x)
- 3y2=4x(1-x)
- 9y2=8x(x-1)
- 3y2=4x(x-1)
Answer
(1) 9y2=8x(1-x)
Solution
[figure]
Directrix x= ae=9, e= 13 a=3
b2=a2 (1-e2 )=9 89=8
E: x29+ y28=1, focus P(ae, 0)=(1, 0)
Let M(h, k) be the mid point of AB. Chord with mid point M: T=S1
hx9+ ky8= h29+ k28
It passes through P(1, 0):
h9= h29+ k28
8h=8h2+9k2 9k2=8h(1-h)
Locus: 9y2=8x(1-x)
Answer: (1)