an=n-1+1/2^n
Sn=n(n-1)/2+1-1/2^nA.1/2^n+(n^+n)/2B.-1/2^+(n^+n)/2+1 C.-1/2^+(n^+n)/2D.-1/2^(n+1)+(n^-n)/2 选哪个因该是-1/2^n+(n^2-n)/2+1上面四个答案在n=1时没有一个是=1/2的