`=> x + 5 in Ư(30)`
`=> x + 5 in {(+-1, +-2, +-3, +-5, +-6, +-10, +-15, +-30)`
`=> x in {(-4, -6, -7, -3, -2, -8, 0, -10, 1, -11, 5, -15, -20, 10, -35, 25}`.
ta cóx+5∈Ư(30)⇒x+5∈Ư(30)
suy ra x+5∈{(±1,±2,±3,±5,±6,±10,±15,±30)⇒x+5∈{(±1,±2,±3,±5,±6,±10,±15,±30)
⇒x∈{(−4,−6,−7,−3,−2,−8,0,−10,1,−11,5,−15,−20,10,−35,25}