Đáp án:
` => x/19 = y/21`
` = (x-y)/(19-21) = 4/-2 = -2`
` => x = -38 ; y = -42`
`b)`
` x/3 = y/7`
Đặt ` x/3 y/7 = k`
` => x= 3k ; y = 7k`
` => xy = 84 = 3k . 7k = 84`
` => 21k^2 = 84`
` => k = ± 2`
` => x = ±6 ; y = ±14`
`c)`
` => x/3 = y/5 = z/-2 = 5x/15 = 3z/-6 = (5x-y+3z)/(15-5-6) = 124/4 = 31`
` => x =93 ; y = 155; z = -62`