1/n(n+1)(n+2)=1/2*[1/n-2/(n+1)+1/(n+2)]
原式=1/2*(1-2*1/2+1/3+1/2-2*1/3+1/4+.+1/9-2*1/10+1/11)
=1/2*(1-1/2-1/10+1/11)=27/110