Đáp án:
Giải thích các bước giải:
`a,` -Gọi `x=4k ( k in NN^** )`
`y=7k ( k in NN^** )`
-Ta có: `4k - 7k = -15`
`-> -3k = -15`
`-> k = 5`
`-> x = 5 . 4 = 20`
`-> y = 5 . 7 = 35`
`b,` -Gọi `x = 4k ( k in NN^** )`
`y = 2k ( k in NN^** )`
`-> 4y=8k`
`z = 7k ( k in NN^** )`
-Ta có: `4k - 8k + 7k = 21`
`-> 3k = 21`
`-> k = 7`
`-> x = 7 . 4 = 28`
`-> y = 7 . 2 = 14`
`-> z = 7 . 7 = 49`
`c, x/3 = y/5 -> 4x/12 = y/20`
`3x=4z -> x/4=z/3 -> x/12 = z/9`
`-> x/12 = y/20 = z/9`
-Gọi `x = 12k -> 4x = 48k`
`y = 20k -> 3y = 60k`
`z = 9k -> 2z=18k`
-Với `k in NN^**`
-ta có: `48k - 60k - 18k = -24`
`-> -30k = -24`
`-> k = 4/5`
`-> x = 4/5 . 48 = 192/5`
`-> y = 4/5 . 60 = 48`
`-> z = 4/5 . 18 = 72/5`
