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Question

Given that 1+x+x2+xn1=1xn1x,x1, Using this, find the sum of the series 1+2x+3x2+(n1)xn2

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Solution

1+x+x2++xn1=1xn1x
Differentiating on both sides with respect to x
1+2x+3x2++(n1)xn2=(1x)(nxn1)+1xn(1x)2=(n1)xnnxn1+1(1x)2

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