Let P(x1, y1, z1) and Q(x2, y2, z2) be two points and let R(x, y, z) be a point on PQ dividing it in the ratio m:n. Prove that x=mx2+nx1m+n, y=my2+ny1m+n and z=mz2+nz1m+n.
If l, m, n are the direction cosines of the normal to the plane and p be the perpendicular distance of the plane from the origin, then the equation of the plane is: