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If the lines y = a_1*x + b_1, y = a_2*x + b_2 are not perpendicular then acute angle φ between the lines can find from the formula. tg φ = a_1 - a_2 / 1 + a_1 * a_2. Example. The angle between the lines y = -2x + 6, y = 3x + 1 equals 45°.