1/[n(n+1)]=1/n-1/(n+1)套用这个公式1除以（n+3）(n+4)+1除以(n+4)(n+5)+、、、1除以(n+10)(n+11)=1/(n+3)-1/(n+4)+1/(n+4)-1/(n+5)+...+1/(n+10)-1/(n+11)=1/(n+3)-1/(n+11)=(n+11-n-3)/[(n+3)(n+11)=8/(n^2+14n+33...