`a)4/5+x=8/10`
`=>x=8/10-4/5`
`=>x=4/5-4/5`
`=>x=0`
Vậy `x=0`
__
`b)1/7-(2/3-x)=-4/7`
`=>2/3-x=1/7-(-4/7)`
`=>2/3-x=1/7+4/7`
`=>2/3-x=5/7`
`=>x=2/3-5/7`
`=>x=14/21-15/21`
`=>x=-1/21`
Vậy `x=-1/21`
14:
a: CB=10-4=6cm
b: CM=BM=6/2=3cm
c: AM=AB-BM=10-3=7cm