Goi I la trug djem cua AB,=>GI/SI=1/2.Ke MN//DC =>BC=2BN 
de cm GM//(DCS) thj ta cm (MNG)//(SCD) 
Ta thay MN//DC(1) nen tu G ke du0g thag //AB cat SB,SA lan lu0t taj P va Q =>mp(NMG) cat chop S.ABCD theo thjet dien la hjh thag MNPQ ta co GI/SI=BP/BS=BN/BC=1/3 
=>PN//SC(2) 
tu 1,2=>(MNPQ)//(SCD) =>GM//(SCD)