1/3+1/3*5+1/5*7+.+1/19*21
问题描述：
1/3+1/3*5+1/5*7+.+1/19*21
=1-1/3+1/3-1/5+1/5+.-1/21
=1-1/21
=20/21
1是怎么来的?
答
1/3+1/3*5+1/5*7+.+1/19*21
=1/2*(1-1/3+1/3-1/5+1/5+.-1/21)
=1/2*(1-1/21)
=10/21