Question

x+2x+3x+……nx^n(x的n次方)（x属于R）谢谢了,

Answer 1

sn=x+2x+3x+……nx^n Xsn=x^2+2x^3+…nx^(n+1) xSn-Sn=(x^2+2x^3+…nx^(n+1))-(x+2x^2+3x^3+…nx^n) =-x-x^2-x^3-..-x^n+nx^(n+1) =-x(x^(n+1)-1)/(x-1)+nx^(n+1)