Question

If P is a point on an ellipse 5x² + 4y² = 80 whose foci are S and S'. Then PS + PS' = ........

(a) 4√5

(b) 4

(c) 8

(d) 10

Answer 1

The correct option (a) 4√5 

Explanation:

P is point on ellipse 5x² + 4y² = 80 and foci are S and S'.

i.e.       (x²/16) + (y²/20) = 1

⇒         a = 4 and b = √20

Now PS + PS' = 2b since major axis of ellipse is along y axis 

∴   PS + PS' = 2b = 2√20 = 4√5.