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若x=1 則s=1+2+……+n=n(n+1)/2 若x不等於1 則xs=x+2x^2+3x^3+……+(n-1)x^(n-1)+nx^n s-xs=1+x+x^2+x^3+……+x^(n-1)-nx^n =1*(1-x^n)/(1-x)-nx^n 所以s=(1-x^n)/(1-x)^2-nx^n/(1-x)
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