Angle between Two Lines
The angle between two lines is the smaller of the angles formed by the intersection of the two lines. The angle can be obtained from:
1. Their slopes
2. Their direction vectors
Find the angle between the lines r and s, if their directional vectors are : 
= (−2, 1) and 
= (2, −3).
Given the lines r ≡ 3x + y - 1 = 0 and s ≡ 2x + my - 8 = 0, calculate the value of m so that the two lines form an angle of 45°.
