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Question

Let S, S be the foci of the ellipse x2a2+y2b2=1 whose eccentricity is 'e'. P is a variable point on the ellipse. Consider the locus of the incentre of the PSS

The locus of the incentre of the PSS is

A
ellipse
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B
hyperbola
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C
parabola
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D
circle
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Solution

The correct option is A ellipse

Let the coordinates of P be
(acos θ, bsin θ)
Here,
SP= Focal distance of point P
So,
SP=aae cos θ
SP=a+ae cos θ
SS=2ae

If (h,k) are the coordinates of the incenter of ΔPSS, then
h=2ae(a cos θ)+a(1e cos θ)(ae)+a(1+e cos θ)ae2ae+a(1e cos θ)+a(1+e cos θ) =ae cos θ(i)
and
k=2ae(b sin θ)+a(1e cos θ)×0+a(1+e cos θ)×02ae+a(1e cos θ)+a(1+e cos θ) =eb sin θ(e+1)(ii)
From (i) and (ii),
hae=cosθk(e+1)eb=sinθ
Eliminating θ, we get
x2a2e2+y2(be(e+1))2=1
Which clearly represents an ellipse.


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