Let the lines represented by S = 0 be l1x + m1y + n1 = 0 and l2x m2y + n2 = 0. Therefore
Therefore, the lines represented by ax2 + 2hxy + by2 = 0 are l 1x + m1y = 0 and l2x + m2y = 0 which are, respectively, parallel to the l1x + m1y + n1 = 0 and l2x + m2y + n2 = 0.