# Math Help - Angle between 2 lines proof

1. ## Angle between 2 lines proof

Let L1 and L2 be two lines in the plane, with equations y = m1x + c1 and y = m2x + c2 respectively. Suppose that they intersect at an acute angle θ. Show that

$\tan\theta = |\frac{m_1-m_2}{1+m_1m_2}|$

Help please, I've drawn a diagram but I don't really know how to proceed.

2. ## Re: Angle between 2 lines proof

Originally Posted by elieh
Let L1 and L2 be two lines in the plane, with equations y = m1x + c1 and y = m2x + c2 respectively. Suppose that they intersect at an acute angle θ. Show that

$\tan\theta = |\frac{m_1-m_2}{1+m_1m_2}|$

Help please, I've drawn a diagram but I don't really know how to proceed.
$if\alpha_1and\alpha_2$ are inclinations of the lines use the fact that
$m_1=\tan\alpha_1&m_2=\tan\alpha_2$