1) Let S and S' be the foci of an ellipse and B be one end of its minor axis. If SBS' is an isosceles right-angled triangle then the eccentricity of the ellipse is A) 1√2 B) 12 C) √32 D) 13 Answer: Option AExplanation: We have, SBS' is an isosceless right angled triangle. ∴ SS′2=SB2+S′B2 ⇒ (2ae)2=b2+(ae)2+b2+(ae)2 ⇒ 4(ae)2=2(b2+(ae)2) ⇒ (ae)2=b2 ⇒ e2=b2a2 ⇒ 1−e2=1−b2a2 ⇒ 1−e2=e2 [∵e=√1−b2a2] ⇒ 2e2=1 ⇒ e=1√2