(3/4-3/5）÷7/10
=(15/20-12/20)x10/7
=3/20 x 10/7
=3/14
（2/3+2/9）÷20/21
=(6/9+2/9)x21/20
=8/9 x 21/20
=2/3 x 7/5
=14/15

1-5/14÷5/7
=1-5/14 x 7/5
=1-1/2
=1/2

1/4÷（2/3+1/2）×4/5
=1/4÷7/6 x 4/5
=6/28 x 4/5
=6/35确定对吗?确定全死算？你要把括号外的乘与括号内的先约分？那样也不简单，因为这些数很小，通分很简单，如数字大，宜先约分化简再算加减。