A plano convex lens of thickness 4 cm is found to be 3 cm thick if viewed through its plane face. But when viewed through its curved surface it is found to be 25 cm thick. Calculate the focal length of the lens.

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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?