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I am trying to find angle between two lines represented by a homogeneous equation

The equation is : $ 7x^2 + 4xy + 4y^2 = 0 $

When i use the standard formula

$ \theta = \arctan \frac {2 \sqrt {h^2 - ab}} {a + b} $

But $ h^2 - ab $ is negeative and i cannot find the angle.

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
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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
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1 Answer 1

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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