KVPY2020MathematicsEllipse
Let x 2 a 2 + y 2 b 2 = 1 ( b < a ) , be an ellipse with major axis A B and minor axis C D . Let F 1 and F 2 be its two foci, with A , F 1 ,   F 2 ,   B in that order on the segment A B . Suppose ∠ F 1 C B = 90 ° . The eccentricity of the ellipse is
Options
- A3 - 1 2
- B1 3
- C5 - 1 2
- D1 5
Correct answer
C. 5 - 1 2
Step-by-step solution
b − a e × b a = − 1 ⇒ b 2 = a 2 e ⇒ a 2 1 − e 2 = a 2 e ⇒ e 2 + e − 1 = 0 ⇒ e = − 1 + 5 2