`3.`
`a) x/(1,2)=5/6`
`6x=5.1,2`
`6x=6`
`x=6:6`
`x=1`
Vậy `x=1`
`b) 5/9:x=7/4:3/10`
`5/9:x=35/6`
`x=5/9:35/6`
`x=2/21`
Vậy `x=2/21`
`4.`
`(x-y)/(x+2y)=3/4`
`4(x-y)=3(x+2y)`
`4x-4y=3x+6y`
`4x-3x=6y+4y`
`x=10y`
`x/y=10`
`5^(**).`
`(a+b)/(c+d)=(b+c)/(d+a)`
`=> (d+a)(a+b)=(b+c)(c+d)`
`=> ad+a^2+bd+ab=bc+c^2+bd+cd`
`=> ad+a^2+ab=cd+c^2+cb`
`=> a=b`