Angle between 2 lines proof

**elieh** · August 4th 2011, 10:17 AM

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.
Thanks in advance

**Also sprach Zarathustra** · August 4th 2011, 10:29 AM

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.
Thanks in advance

Here:

Angle Between Two Lines