How can you evaluate ∑∞n=1nxn−1=1(1−x)2 without relying on the fact that it's the derivative of ∑∞n=1xn=11−x?
Answer
a=1+2x+3x2+4x3+5x4+6x5+...|x|<1multiply a by x xa=x+2x2+3x3+4x4+5x5+6x6+...now subtract a and ax a−xa=1+(2x−x)+(3x2−2x2)+(4x3−3x3)+...a(1−x)=1+x+x2+x3+x4+x5+...a(1−x)=11−xa=1(1−x)2
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