1/(n*(n+1)*(n+2))=1/2*(1/(n*(n+1))-1/((n+1)*(n+2)))上式=1/2*(1/(12*13)-1/(13*14)+1/(13*14)-1/(14*15)+1/(14*15)-1/(15*16)+1/(15*16)-1/(16*17))=1/2*(1/(12*13)-1/(16*17))=29/21216