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Sn=1+2x+3x^2+-----+nX^(n-1)= xSn=x+2x^2+...+(n-1)x^(n-1)+nx^n 2つの減算: (1-x)Sn=1+x+x^2+..x^(n-1)-nx^n=(1-x^n)/(1-x)-nx^n Sn=(1-x^n)/(1-x)^2-nx^n/(1-x)
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