三、怎么算简便就怎样算.1/4+3/7+4/7+3/4 2.8/9-1/4+1/2 3.3/8+5/6+5/8 4.2-

1/4+3/7+4/7+3/4
=1/4+3/4+3/7+4/7
=(1/4+3/4)+(3/7+4/7)
=1+1==2
2.8/9-1/4+1/2
=8/9+(1/2-1/4)
=8/9+1/4
=32/36+9/36
=41/36
3.3/8+5/6+5/8
=(3/8+5/8)+5/6
=1+5/6
=11/6
4.2-1/8-7/8
=2-(1/8+7/8)
=2-1
=1
5.1+3/4-7/12
=1+9/12-7/12
=1+1/6
=7/6
6.5/8+1/4-3/4
=5/8-1/2
=5/8-4/8
=1/8