裂项求和
1/1*3+1/3*5+1/5*7+1/7*9+1/9*11+1/11*13+1/13*15+1/53*55
=1/2*(1-1/3+1/3-1/5+...+1/13-1/15..-1/55)
=1/2*(1-1/55)
=1/2*54/55
=27/55中间是省略号啊！！！！！！！！对呀！