[(→a+→b).(→b+→c),(→c+→a)]=(→a+→b).[(→b+→c)×(→c+→a)]=(→a+→b).[→b×→c+→b×→a+→c×→c+→c×→a]=(→a+→b).(→b×→c+→b×→a+→c×→a)=→a.(→b×→c)+0+0+0+0+→b.(→c×→a)=[→a→b→c]+[→b→c→a]=[→a→b→c]+[→a→b→c]=2[→a→b→c]proved
now given vector will be coplanar only when →a,→b,→carecoplanar,sothat[→a→b→c]=0