Given the slope of two intersecting lines, the formula for calculating the angle between them (how do you get it?)

Given the slope of two intersecting lines, the formula for calculating the angle between them (how do you get it?)

Tan θ = ∣ (K2 - K1) / (1 + k1k2) ∣. Note that the tangent of the acute angle formed by the intersection of two straight lines is calculated
When two straight lines intersect at a point and intersect with X axis at two points m and N respectively, the inclination angle of a straight line is α and that of a straight line is β,
In the triangle Mon, the inclination angle β is an external angle of the triangle, which is equal to the sum of the two internal angles not adjacent to it. In this case, the angle between the acute angles of the two lines is equal to α - β
So to calculate this, with the help of its tangent value, first find the tangent value, ∣ Tan (α - β) ∣ = ∣ (Tan α - Tan β) / (1 + Tan α Tan β) ∣ = ∣ (K2 - K1) / (1 + k1k2) ∣

How to find a linear equation for 2-point coordinates?

Two point formula of linear equation
(y-y1)/(x-x1)=(y-y2)/(x-x2)

On the linear equation: to prove the midpoint coordinate formula

Given p (x1, Y1) and Q (X2, Y2), let the midpoint of PQ be m (x, y)
Then, from vector PM = vector MQ, we can get
（x-x1,y-y1）=(x2-x,y2-y)
So x-x1 = x2-x, Y-Y1 = y2-y
That is, x = (x1 + x2) / 2, y = (Y1 + Y2) / 2