The centre of a circle C is at the centre of the ellipse E: x2a2 + y2b2 = 1, a > b. Let C pass through the foci F1 and F2 of E such that the circle C and the ellipse E intersect at four points. Let P be one of these four points. If the area of the triangle PF1F2 is 30 and the length of the major axis of E is 17, then the distance between the foci of E is :
- 12
- 26
- 13
- 132
Answer
(3) 13
Solution
C has centre O and passes through F1, F2 F1F2 is a diameter of C.
F1PF2 = 90 (angle in a semicircle)
Let PF1 = r1, PF2 = r2 and F1F2 = 2c.
P on E r1 + r2 = 2a = 17
Area = 12r1r2 = 30 r1r2 = 60
Pythagoras: r12 + r22 = (2c)2
(2c)2 = (r1 + r2)2 - 2r1r2 = 289 - 120 = 169
2c = 13
Answer: (3)