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 A. Show that

tan A = (m1-m2)/(1+m1m2) <-- modulus of this

The lines and modulus shows that I should use the formula for cos A with the dot product etc. But I am not sure what the dot product will be.

Any hints?