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ZANCAI(Edstrayer´ú·¢): ½ð±Ò+5, thank 2020-09-14 03:26:04
¦² x^n = 1/(1-x)
d/dx ¦² x^n = d/dx[1/(1-x)]
¦² nx^(n-1) = 1/(1-x)^2
1+2x+3x^2+......+nx^n-1+... = 1/(1-x)^2
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