1×3分之1＋3×5分之1＋5×7分之1...＋17×19分之1＋19× 21分之1 =1/2（1-1/3）+1/2(1/3-1/5)+1/2(1/5-1/7)+……+1/2(1/17-1/19)+1/2(1/19-1/21)=1/2(1-1/21)=10/21
