1/1×3+1/3×5+1/5×7+1/7×9+...+1/19×+21
=(1-1/3)/2+(1/3-1/5)/2+(1/5-1/7)/2+(1/7-1/9)/2+...+(1/19-1/21)/2
=(1-1/3+1/3-1/5+1/5-1/7+...+1/19-1/21)/2
=(1-1/21)/2
=10/21