Clearly the coordinates of both points A and B satisfy the line equation lx + my =1 as well as S = 0 and hence points A and B satisfy S' = 0. Also (0, 0) satisfies S' = 0. That is, S' = 0 passes through A, B and origin (see Fig.). On simplification, we can see that S' = 0 is a homogeneous equation of second degree representing pair of lines, which are nothing but the lines (bar)OA and (bar)OB.