由裂项公式1/[n(n+1)(n+2)]=1/2[1/n(n+1) -1/(n+1)(n+2)]原式= 1/2[1/(1×2）-1/（21×22）]=115/462
1/1*2*3+1/2*3*4+1/3*4*5+1/4*5*6+……+1/20*21*22=0.5(1/1*2-1/2*3+1/2*3-1/3*4....)=0.5*(0.5-1/21*22)
