利用公式：2/n(n+1)(n+2)=1/n(n+1)-1/(n+1)(n+2) 1/(4×5×6)+1/(5×6×7)+…+1/(98×99×100) =1/2[1/4*5-1/5*6+1/5*6-1/6*7+.+1/98*99-1/99*100] =1/2(1/20-1/9900) =73/2475