Ta có: `VT= \vec{AI} + \vec{BM} +\vec{CN} `
`= \vec{AC} + \vec{CI}+ \vec{BA}+\vec{AM} + \vec(CB)+\vec(BN) `
`= (\vec{AC} + \vec{BA}+ \vec(CB))+(\vec{AM} + \vec{CI}+\vec(BN)) `
`= \vec{AM} + \vec{CI}+\vec(BN)`
`=VP (đpcm)`
Vậy: ` \vec{AI} + \vec{BM} +\vec{CN} = \vec{AM} + \vec{CI}+\vec(BN)`