用简便方法计算1-3/2+5/6-7/12+...-19/90+21/110

(1-3/2)+(5/6-7/12)+(9/20-11/30)+(13/42-15/56)+(17/72-19/90)+21/110
=(-1/2+1/4+1/12+1/24+1/40)+21/110
=-1/10+21/110
=-1/11
应该还有更简便的数理关系,但我还没看出来