Đáp án:
`a)`
` x/3 = y/5 =z/8 = (2x)/6 = (3y)/15 = (2x+3y)/(6+15) = 39/21= 13/7`
` => x= 13/7 *3 = 39/7`
` => y= 13/7 * 5= 65/7`
` => z = 13/7 *8 = 104/7`
`b)`
` x/9 = y/12 = z/20 = (2x)/18 = (3y)/15 = (2x-3y+z)/(18-15+20) = 6/33`
` => x= 6/33 * 9 = 54/33`
` => y= 6/33 *12 = 72/33`
` => z = 6/33 *20 = 120/33`
`c)`
` => x/12 = y/-3= (2y)/(-6) = (x-2y)/(12+6) = -49/18 `
`=> x =-98/3`
` => y = 49/6`
`d)`
` => x/4 = y/21 = (2x)/8 = (2x-y)/(8-21) = 34/-13 = -13/34`
` => x= -26/17`
` => y= 273/34`
