Let →r be the only point of intersection of the planes →r.→a=P1,→r.→b=P2,→r.→c=P3 then
are non – coplanar
Let →r=λ1(→b×→c)+λ2(→c×→a)+λ3(→a×→b)→r.→a=λ1[→a→b→c]⇒P1=λ1[→a→b→c]λ1=P1[→a→b→c] similirly λ2 P2[→a→b→c], λ3 P1[→a→b→c]