1/n(n+1)(n+2) = 1/(n+1) {（1/2）[ 1/n - 1/(n+2) ]}=（1/2）[ 1/(n+1)1/n - 1/(n+1)1/(n+2) ]=（1/2）[ (1/n - 1/(n+1)) - (1/(n+1)-1/(n+2))]=（1/2）[ 1/n - 2/(n+1)) +1/(n+2) ]1/1*2*3+1/2*3*4+…+1/n(n+1)(n+...