1/1x3+1/2x4+1/3x5+……+1/9x11
＝1/2(1-1/3+1/2-1/3+1/3-1/5+...+1/7-1/9+1/8-1/10+1/9-1/11)
=1/2(1-1/3+1/2+1/8-1/10-1/11)