2/1×3+2/3×5+2/5×7+···+2/19×21
问题描述：
2/1×3+2/3×5+2/5×7+···+2/19×21
答
2/1×3+2/3×5+2/5×7+···+2/19×21
=(1/1-1/3)+(1/3-1/5)+(1/5-1/7)+···++(1/19-1/21)
=1-1/21
=20/21