(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確定對(duì)嗎?確定全死算？你要把括號(hào)外的乘與括號(hào)內(nèi)的先約分？那樣也不簡(jiǎn)單，因?yàn)檫@些數(shù)很小，通分很簡(jiǎn)單，如數(shù)字大，宜先約分化簡(jiǎn)再算加減。