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=?
答
20/21
答
2/1*3=1-1/3
2/3*5=1/3-1/5
2/5*7=1/5-1/7
......
2/19*21=1/19-1/21
所以
2/3*5+2/5*7+......+2/17*19+2/19*21
=1-1/3+1/3-1/5+1/5-1/7+...+1/19-1/21
=1-1/21
=20/21
答
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
答
2/1*3+2/3*5+2/5*7+......+2/17*19+2/19*21
=1-1/3+1/3-...+1/19-1/21
=20/21
谢谢采纳···