Monday, February 1, 2016

calculus - Evaluate sumin=1nftynxn1


How can you evaluate n=1nxn1=1(1x)2 without relying on the fact that it's the derivative of n=1xn=11x?


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 axa=1+(2xx)+(3x22x2)+(4x33x3)+...a(1x)=1+x+x2+x3+x4+x5+...a(1x)=11xa=1(1x)2


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