1/2x3+1/3x4+1/4x5+...+1/(n+1)(n +2）=1/2-1/3+1/3-1/4+1/4-1/5+....+1/(n+1)-1/(n +2）=1/2-1/(n +2）=n/2(n +2）
1/2-1/n+1
