a tangent to ellipse x^2/a^2+y^2/b^2=1 cut the ellipsex^2/c^2+y^2/d^2=1 at P and Q. If tangent to the ellipse at Pand Q intersect at right angle. Then prove that a^2/c^2+b^2/d^2=1.

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Sum of series 2[n/2] ! [n/2]!/ n! [2Co-2C2+3C2....+[-1]^n[n+1]2Cn] where n is an even positive integer. Kindly provide solution of this question and how to generalise this. How do we know that which expansion will give this?

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