楼上2位合一起为正解1/1*2*3+1/2*3*4+1/3*4*5+1/4*5*6={（1/1*2-1/2*3）+（1/2*3-1/3*4）+.+[1/n*(n+1)-1/(n+1)*(n+2)]}/2=[1/2-1/(n+1)*(n+2)]/2=[(n+1)(n+2)-2]/4(n+1)(n+2)