A straight line intersects a straight line at a point. Rotate the angle counterclockwise around this point, which is called the angle from to. Similarly, it is called the angle from to. So, tan=(k? -k？ )/( 1+k？ k？ )。

The included angle formula assumes that the slopes of straight lines l 1 and l2 exist, which are k 1 and k2 respectively, and the turning angle of l 1 to l2 is θ, then Tan θ = (k2-k1)/(1k2) L1.

Slope formula of straight lines: k = (Y2-Y 1)/(X2-X 1) Note: The included angle between two straight lines refers to the angle formed by two straight lines less than 90. Obviously, the "angle" in the included angle formula is not all the included angle between two straight lines.

Significance of inclination angle:

In the plane rectangular coordinate system, when the straight line L intersects the X axis, we take the X axis as the benchmark, and the minimum positive angle when the X axis rotates counterclockwise (positive direction) around the intersection point and coincides with the straight line L is recorded as α, which is called the inclination angle of the straight line L. When L is parallel or coincident with the X axis, we specify that its inclination angle is zero.

A straight line with an inclination angle other than 90 is called the slope of this straight line. The slope of a straight line is often expressed by k, and the straight line coincident with the Y axis has no slope.