Bài 9:
A=(đề bài)
=-7/21+4/3
=-7/21+28/21
=21/21
=1
B=(đề bài)
=2/15+-1/9
=6/45+-5/45
=1/45
C=(đề bài)
=-1/5+(3/12+-3/4)
=-1/5+(3/12+-9/12)
=-1/5+-6/12
=-1/5+-1/2
=-2/10+-5/10
Bài 10:
a/đề bài
=(4/20+3/20)+(6/15+-3/5)+(2/21+-10/21)+16/42
=7/20+(6/15+-9/15)+-8/21+8/21
=7/20+-3/15
=21/60+-12/60
=9/60=3/20
b/đề bài
=21/23+126/93+-21/23+-125/143
=21/23+-21/23+125/93+-125/143
=125/93+-125/143
=17875/13299+-11625/13299
=6250/13299
Bài 11:
a/đề bài
=14/6+3/6--3/70
=17/6--3/70
=1190/420--18/420
=1208/420=302/105
b/đề bài
=5/12--3/16+12/16
=5/12+3/16+12/16
=5/12+15/16
=20/48+45/48
=65/48
Chúc bạn học tốt :33
