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Q.

Show that the locus of the feet of the perpendicular drawn from foci to any tangent to the ellipse is lies on a circle concentric with the ellipse. (auxiliary circle)

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Detailed Solution

Let the equation of the ellipse be S=x2a2+y2b21=0
Question Image
Let P(x1, y1) be the foot of the perpendicular drawn from either of the foci to a tangent.
The equation of the tangent to the ellipse S=0 is y=mx±a2m2+b2 ...(1)
The equation to the perpendicular from either foci (±ae,0) on this tangent is
y=1m(x±ae)(2)
As P is the point of intersection of (1) & (2)
we have
y1=mx1±a2m2+b2,y1=1mx1±aey1mx1=±a2m2+b2,my1+x1=±aey1mx12+my1+x12=a2m2+b2+a2e2y12+m2x122x1y1m+m2y12+x12+2x1y1m =a2m2+a21e2+a2e2
x12m2+1+y121+m2=a2m2+a2x12+y12m2+1=a2m2+1x12+y12=a2
P lies on x2+y2=a2 which is a circle with centre at the origin, the centre of ellipse.

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