For two lines with slopes m1 and m2, the acute angle between them satisfies tan(theta) = |m1 - m2|/(1 + m1m2). This formula is valid when m1m2 is not equal to -1. When 1 + m1m2 becomes negative, the computed angle is obtuse; take the supplement for the acute angle. Two lines are parallel when m1 = m2 (same slope) and perpendicular when m1m2 = -1 (product of slopes equals -1). In general form, lines a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0 are parallel when a1/a2 = $\frac{b1}{b2}$ and perpendicular when a1a2 + b1b2 = 0. These conditions are among the most frequently tested concepts in JEE.