1/1*5+1/5*9+1/9*13+1/13*17+1/17*21= 小学简便运算
问题描述：
1/1*5+1/5*9+1/9*13+1/13*17+1/17*21= 小学简便运算
答
1/1*5+1/5*9+1/9*13+1/13*17+1/17*21
=1/4*(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)
=1/4*(1-1/21)
=1/4*20/21
=5/21