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Question

The coefficient of xn in (1+x+2x2+3x3+.....+nxn)2 is ?

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Solution

S=1+x+2x^2+3x^3......+nx^n
xs=x+x^2+2x^3+.......+nx^(n+1)
(1-x)s=1+x+x^2+.....+x^n
-nx^(n+1)-x=((1-x^(n+1))/(1-x) )-nx^(n+1) - x
s= (1÷(1-x)^2)-(x÷(1-x))
= (1-x+x^2)÷(1-x)^2
coefficient of x^n is (1-x+x^2)^2 (1-x)^(-4)

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