2/1*3+2/3*5+2/5*7+.+2/17*19+2/19*21=?

问题描述:

2/1*3+2/3*5+2/5*7+.+2/17*19+2/19*21=?

2/1*3+2/3*5+2/5*7+.+2/17*19+2/19*21
=1-1/3+1/3-1/5+1/5-1/7+..+1/17-1/19+1/19-1/21
=1-1/21
=20/21