Aha, the slope is m1 and m2? Then the angle's tangent is |(m1 - m2) / (1 + m1*m2)|. They teach that in every city book. But listen, two lines crossing make four angles. Your formula gives the acute one, usually. Life isn't just one angle. In my orchard, the property line and the old fence have a slope, but the real fight is the acute angle the city's new road makes, trying to cut my land. That's the angle that keeps me up.
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