1/
Ta có :
`(a+b)/(c+d) =(b+c)/(d+a)`
`=> (a+b)/(c+d) +1 =(b+c)/(d+a) +1`
`=> (a+b+c+d)/(c+d) = (b+c+d+a)/(d+a)`
`=> (a+b+c+d)/(c+d) -(b+c+d+a)/(d+a) =0`
`=> (a+b+c+d)(1/(c+d) - 1/(d+a) ) =0`
Mà ` a+b+c+d \ne 0`
`=> 1/(c+d) - 1/(d+a) =0`
`=> 1/(c+d) =1/(d+a)`
`=> c+d =a+d`
`=> a =c` (ĐPCM)
2/
`(3x-y)/(x+y) =3/4`
`=> 4(3x-y) = 3(x+y)`
`=> 12x - 4y =3x + 3y`
`=> 12x - 3x = 3y +4y`
`=> 9x = 7y`
`=> x/y = 7/9`
Vậy `x/y = 7/9` .
