calculate the angle between a line and x-axis

问题

I have two points in my coordinate system (x,y) that I want to know the angle of their line and x-axis.

I use swift for solving this but I can't get the angle.

I need this angle in radians to use it in the following equation:

(x0 + r cos theta, y0 + r sin theta)

r : radius of circle

回答1:

If you have two points, (x0, y0) and (x1, y1), then the angle of the line joining them (relative to the X axis) is given by:

theta = atan2((y1 - y0), (x1 - x0))

回答2:

The angle between a line, for reference, let's call this A, defined by two points p1=(x1,y1),p2=(x2, y2) and the x-axis is related to finding the slope/ gradient of the line, A.

# To solve a problem you sometimes have to simplify it and then work up to the full solution"

Let's start by obtaining the gradient of the line A.

The gradient of line A:

slope = (y2 - y1)/(x2 - x1)

for a straight line, that makes an angle theta with the x-axis tan(theta) = slope = (change in y) / (change in x)

Therefore, theta = tan_inverse (slope)

theta = atan(slope)

来源：https://stackoverflow.com/questions/41855261/calculate-the-angle-between-a-line-and-x-axis