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(1)
consider
(1)
1/(1-x) = 1+x+x^2+...
两边取导
1/(1-x)^2 = 1+2x+3x^2+...
x/(1-x)^2 = x+2x^2+3x^3+..
两边取导
[(1-x)^2 + 2x(1-x)]/(1-x)^4 = 1+2^2.x +3^3.x^2+...
(1-x^2)/(1-x)^4 = 1+2^2.x +3^3.x^2+...
(1+x)/(1-x)^3 = 1+2^2.x +3^3.x^2+...
∑(n:1->∞) n^2.x^(n-1) =(1+x)/(1-x)^3
(2)
1/(1-x) = 1+x+x^2+...
两边取导
1/(1-x)^2 = 1+2x+3x^2+...
1/(1-x)^2 -1 =2x+3x^2+...
∑(n:1->∞)(n+1)x^n = 1/(1-x)^2 -1
consider
(1)
1/(1-x) = 1+x+x^2+...
两边取导
1/(1-x)^2 = 1+2x+3x^2+...
x/(1-x)^2 = x+2x^2+3x^3+..
两边取导
[(1-x)^2 + 2x(1-x)]/(1-x)^4 = 1+2^2.x +3^3.x^2+...
(1-x^2)/(1-x)^4 = 1+2^2.x +3^3.x^2+...
(1+x)/(1-x)^3 = 1+2^2.x +3^3.x^2+...
∑(n:1->∞) n^2.x^(n-1) =(1+x)/(1-x)^3
(2)
1/(1-x) = 1+x+x^2+...
两边取导
1/(1-x)^2 = 1+2x+3x^2+...
1/(1-x)^2 -1 =2x+3x^2+...
∑(n:1->∞)(n+1)x^n = 1/(1-x)^2 -1
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