Dear Rahul
 
Use mid point of chord of contact formula
 
T= S1
 
Hope it helps
 
Regards
Arun (askIITians forum expert)
						

							
Let any point on the given circle be O(d cos, d sin)
and midpoint of chord PQ of ellipse be M(h,k).
Equation of chord of contact PQ with O as an external point is
T = 0 w.r.t. the ellipse x2/a2 + y2/b2 – 1 = 0.
  dcosx/a2 + dsiny/b2 – 1 = 0  i.e. 
dcosx/a2 + dsiny/b2   =  1                          ….(1)    is the equation of PQ.
 
Also, considering midpoint M, equation of chord PQ is
T = S1 w.r.t. the ellipse x2/a2 + y2/b2 – 1 = 0.
 hx/a2 + ky/b2 – 1 = h2/a2 + k2/b2 – 1  i.e.
hx/a2 + ky/b2   =  h2/a2 + k2/b2                 ….(2)    is the equation of PQ.
 
Comparing (1) & (2), we get
h/dcos  =  k/dsin  =  (h2/a2 + k2/b2) / 1
 cos = h/d(h2/a2 + k2/b2)  &  sin = k/d(h2/a2 + k2/b2)
 
 cos2 + sin2 = 1
 [h/d(h2/a2 + k2/b2)]2 + [k/d(h2/a2 + k2/b2)]2 = 1
 h2 + k2 = d2(h2/a2 + k2/b2)2
 
Replace h by x and k by y.
   x2 + y2 = d2(x2/a2 + y2/b2)2 is the required locus.
(You may simplify the above equation to get the desired result.)
Pls approve.