The acute angle between lines with slopes m1 and m2 is given by tan(theta) = |m1 - m2| / (1 + m1m2), valid when m1m2 is not -1. When m1m2 = -1, the lines are perpendicular (theta = 90 degrees). For lines in general form a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0: cos(theta) = |a1a2 + b1*b2| / (sqrt($a1^2$ + $b1^2$) * sqrt($a2^2$ + $b2^2$)).