记
f(x) = ∑(n=2~∞)[nx^(n-1)]/(n-1)
= ∑(n=2~∞)x^(n-1) + ∑(n=2~∞)[x^(n-1)]/(n-1)
= g(x) + h(x),
利用已知级数
∑(n=1~∞)x^(n-1) = 1/(1-x) ,-1