an的通项公式是an=[n(n+1)/2*(n+1)(n+2)/2]/(n+1)=1/4n(n+1)(n+2)=1/4(n^3+3n^2+2n)Sn=1/4[1/4n^2(n+1)^2+3*1/6n*(n+1)(2n+1)+2*n*(n+1)/2]=1/4[1/4n^2(n+1)^2+1/2n*(n+1)(2n+1)+n*(n+1)]不化简了原式=S99=1/4[1/4*9...