-1/1*3-1/3*5-1/5*7-1/7*9-^-1/19*21
问题描述：
-1/1*3-1/3*5-1/5*7-1/7*9-^-1/19*21
答
=-1/2*(1-1/3+1/3-1/5+1/5-1/7+……+1/19-1/21)
=-1/2*(1-1/21)
=-10/21