Let S denote the infinite sum 2+5x +9x2+14x3+20x4+……, where |x|<1 and the coefficient of
Xn-1 is ½ n(n+3), (n=1,2,…..). Then S =

- 2-x/(1-x)3
- 2-x(1+x)3
- 2+x/(1-x)3
- 2+x/(1+x)3

CORRECT ANSWER : 2-x/(1-x)3

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