1/(1*2*3)=(1/2)*(1/1*2-1/2*3)
1/(2*3*4)=(1/2)*(1/2*3-1/3*4)
1/(3*4*5)=(1/2)*(1/3*4-1/4*5)
以此类推,规律就出现了.接下来就看你了.