=（1+3+5+7+9+)+(1－1/3)+(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)
=25+(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=25+10/11
=25又10/11