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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?

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That post is now deleted. :) This is my actual question. ( And looks easier too ) @labbhattacharjee – cipher Nov 30 '12 at 17:57
So, ain't anyone giving answers? – cipher Nov 30 '12 at 17:59
the site somehow defines my problem but i could not get it. Can you please explain it in a clear way @labbhattacharjee . Please – cipher Dec 1 '12 at 15:32
up vote 0 down vote accepted

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

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