【1】5/7除以8/9+7/8除以7/5-5/7
=5/7×9/8+7/8×5/7-5/7
=5/7×（9/8+7/8-1）
=5/7
【2 】8.5*75％+15/2除以4/3
=8.5×3/4+7.5×3/4
=（8.5+7.5）×3/4
=16×3/4
=12