设S=x+2x^2+3x^3+4x^4+······+100x^100,则:
xS=x^2+2x^3+3x^4+4x^5+······+99x^100+100x^101.
∴S-xS=x+x^2+x^3+x^4+······+x^100-100x^101,
∴(1-x)S=x(1-x^100)/(1-x)-100^101,
∴S=x(1-x^100)/(1-x)^2-100x^101/(1-x).
一、当n为偶数时,(-1)^n=1、(-1)^(n+1)=-1,∴此时x=2.
∴x+2x^2+3x^3+4x^4+······+100x^100
=x(1-x^100)/(1-x)^2-100x^101/(1-x)
=2(1-2^100)/(1-2)^2-100×2^101/(1-2)
=2(1-2^100)+100×2^101
=2-2^101+100×2^101
=2+99×2^101.
二、当n为奇数时,(-1)^n=-1、(-1)^(n+1)=1,∴此时x=-2.
∴x+2x^2+3x^3+4x^4+······+100x^100
=x(1-x^100)/(1-x)^2-100x^101/(1-x)
=(-2)[1-(-2)^100]/(1+2)^2-100×(-2)^101/(1+2)
=-2(1-2^100)/9+100×2^101/3
=(300×2^101+2^101-2)/9
=(301×2^101-2)/9