如题所述
原式=1/2(1-1/3)+1/2(1/3-1/5)+1/2(1/5-1/7)+...+1/2(1/19-1/21)..........拆项
=1/2【1-1/3+1/3-1/5+1/5-1/7+...+1/19-1/21】...............去括号合并
=1/2×(1-1/21)
=1/2×20/21
=10/21
what?!
You don't understand?