Here's a classic article from Nonki:
Let's imagine we want to draw an arc.
Assume the center of the arc is located at x = 200, y = 300, and the start angle is a1 = -30 and the end angle = -60.
In mathematics, the y axis goes up. But in Small Basic GraphicsWindow, the y axis goes down. So the sine of an angle becomes the opposite:
Here are the equations: