裂项法：
1又1/3-7/12+9/20-11/30+13/42-15/56+17/72-19/90
=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+1/8+1/9-1/9-1/10
=1-1/10
=9/10