Question
If S and S' are two foci of an ellipse + = 1 (a < b) and P (x1, y1) a point on it, then SP + S' P is equal to-
- 2a
- 2b
- a + ex1
- b + ey1
Hint:
use the definition of an ellipse to find out the value
The correct answer is: 2b
2b
From the definition of ellipse, the sum of distances of a point on the ellipse from the foci is a constant value. This value is equal to length of major axis. In this problem, the major axis is along y axis since b>0
Length of major axis = 2b
Therefore, the value of SP+S’P=2b
the length of major axis of an ellipse is 2a for a horizontal ellipse and 2b for a vertical ellipse. a horizontal ellipse is formed when a>b and a vertical ellipse is formed when b>a. the major axis lies along x axis when a>b and along y axis when b>a.
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