The projection of a segment joining points P(x1,y1,z1) and Q(x2,y2,z2) on a line with direction cosines l,m,n is al+bm+cn. Thena2+b2+c2=

The projection of a segment joining points P(x1,y1,z1)and Q(x2,y2,z2) on a line with direction cosinesl,m,n islx2-x1+my2-y1+nz1-z2Hence,a2+b2+c2=x2−x12+y2−y12+z2−z12=PQ2