I am trying to find angle between two lines represented by the following homogeneous equation: $$ 7x^2 + 4xy + 4y^2 = 0.$$
I tried to use the standard formula $$ \theta = \arctan \left(\frac{2 \sqrt {h^2 - ab}}{a + b}\right),$$ but here $h^2 - ab$ is negative and I cannot find the angle.
Is there any other method to find the angle between them?
After some restless searching. I found out the answer.
Actually the equation i posted does not represent pair of straight line.
For a homogeneous equation to be a pair of straight line (passing through origin), $ h^2 > ab$ is a must