Question

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=?

Answer 1

=1/2*(1-1/3)+1/2*(1/3-1/5)+1/2*(1/5-1/7)+.+1/2*(1/19-1/21) =1/2*(1-1/21) =10/21