làm tiếp

a)7/12+x/15=1/20

x/15=1/20-7/12

x/15=-8/15

x.15=15.-8

x15=-120

x=-120 :15

x=-8∈Z

Vậy x =-8

b)-7/12+8/15=-1/20

-7/x =-1/20-8/15

-7.x=-7/12

x=12∈Z

Vậy x=12

c)9/x+6/35=33/70

9/x=33/70-6/35

9/x=3/10

x=30∈Z

Vậy=30