The homogeneous equation $ax^2$ + 2hxy + $by^2$ = 0 represents a pair of lines through the origin. Individual lines: y/x = (-h +/- sqrt($h^2$ - ab))/b. Conditions: real distinct lines when $h^2$ > ab, coincident when $h^2$ = ab. The lines are perpendicular when a + b = 0. Angle between them: tan(theta) = 2*sqrt($h^2$ - ab)/|a + b|.