2/1÷3÷4/3
=2/1×1/3×3/4
=2/1×（1/3×3/4）
=2×1/4
=1/2
1÷3/7×9/28×4/3
=1×7/3×9/28×4/3
=1×（7/3×4/3）×9/28
=1×28/9×9/28
=1×（28/9×9/28）
=1×1
=1