a)Ta có : ` x \in ƯC(60,84,120 )`
`60 = 2^2 . 3 . 5`
`84 = 2^2 . 3.7`
`120 =2^3 . 3 . 5 `
`ƯCLN ( 60,84,120)= 2^2 . 3 = 12`
`ƯC(60,84,120) = Ư(12) = { 1,2,3,4,6,12}`
mà `x≥6`
Suy ra `x \in {6,12}`
b) Ta có : ` x\in BC(15,14,20)`
`15 = 3.5`
`14 = 2.7`
`20 = 2^2 .5`
`BCNN(15,14,20) = 3 . 5 . 7 . 2^2 =420`
`BC(15,14,20) = B(420) = {0,420,840, 1260,...}
mà `400≤x≤1200`
suy ra `x \in {420,840}`
xin hay nhất !!`AMAX`