Sn=2+5x+8x²+11x的三次方+.+(3n-1)x的n-1次方,x*Sn=2x+5x²+8x^3+11x^4+.+(3n-1)x^n,相减得(1-x)Sn=2+3(x+x^2+.+x^n-1)-(3n-1)x^n,=2+3x*(1-x^(n-1))/(1-x)-(3n-1)x^n,Sn=[(3n-1)x^(n+1)-(3n+2)x^n+x+2]/(1...