1:
1/2+2/3-3/4
=6/12+8/12-9/12
=5/12
7/8-1/6+1/4
=21/24-4/24+6/24
=23/24
1-(3/4-2/9)
=1-(27/36-8/36)
=17/36
9/10-1/5-1/2
=9/10-2/10-5/10
=1/5
1/4+3/7+1/2
=7/28+12/28+14/28
=33/28
7/8-(2/3+1/6)
=21/24-(16/24+4/24)
=1/24
2:
7/8+1/4-1/2
=7/8+2/8-4/8
=5/8
3:
1/2+1/4+1/3
=6/12+3/12+4/12
=13/12
4:
1-1/3-3/5
=1-5/15-9/15
=1/15
(本网页感谢网友qq为99220653纠错)