Let S1,S2 are foci of an ellipse, whose major axis length is 15 units and P be any point on the ellipse such that perimeter of triangle PS1S2 is 20 units. If e is the eccentricity of the ellipse, then 3e=
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Solution
Given, 2a=15
and PS1+PS2+S1S2=20 ⇒2a+2ae=20 ⇒2a(1+e)=20 ⇒15(1+e)=20 ⇒3e=1