求和:Sn=1/1×2×3+1/2×3×4+1/3×4×5+…+1/n×(n+1)×(n+2)

数列通项公式为an=1/n×(n+1)×(n+2) 时,一般用裂项法,就是把每一项分成两个项的差的形式.an=[1/n(n+1) - 1/(n+1)(n+2)]/2 ; （举例：an=1/n(n+1)=1/n - 1/(n+1); an=1/n(n+1)(n+2)(n+3)=[1/n(n+1)(n+2) - 1/(n+1)(n...