Let the length of a latus rectum of an ellipse x^2 a^2+ y^2 b^2=1 be 10 . If its eccentricity is the minimum value of the function f( t)= t^2+ t+ 11…
JEE Main 2025 — Mathematics Coordinate Geometry
2025mcqmedium
Let the length of a latus rectum of an ellipse a2x2+b2y2=1 be 10 . If its eccentricity is the minimum value of the function f(t)=t2+t+1211, t∈R, then a2+b2 is equal to :
Official previous-year question
Held on 7 Apr 2025 · Verified 6 Jul 2026.
Options
A
125
B
126
C
120
D
115
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Solution
Length of LR =a2b2=10⇒5a=b2...(1)
f(t)=t2+t+1211dtdf(t)=2t+1=0⇒t=2−1 Min value of f(t)=(2−1)2+(2−1)+1211=412−1+1211=123−6+11=128=32=ee2=a21−b2⇒94=a21−b2
⇒a2b2=a1−4=a5⇒b2=a5a2...(2)
From (1) & (2) 5a=a5a2⇒a=9,b=45=35∴a2+b2=81+45=126
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