The value of , for which the four points  are coplanar, is

Let     ….(i)

       ….(ii)

     …(iii)

      …..(iv)

From Eq. (i) and (ii), we get

 From Eq. (i) and (iii), we get

Similarly, from Eqs.(i) and (iv), we get

Now, using condition of coplanarity

Applying , we get