Started making it a few months ago, but I forgot that acos(x) gives two different results. Came back to it and somehow realized the problem almost instantly despite having spent hours without finding it in October. Anyways, pressing A moves the A point to your mouse, you can probably guess what pressing B does, and you can drag the big arrow around. The C point will automatically go to the sum point. If the C point is past the edges of the screen then it breaks, so try to avoid that. If you don't know what vectors are, here's an explanation: Vectors are lines between to points, like the lines you see on the screen. Each vector has 3 values: length, direction and angle. If you know the position of both ends of the vector, you can get all those values. In this example, we know where A's coordinates and we know the triangle's, from now on called O, coordinates. With this we can make a 90° triangle using the vector as the hypotenuse, the difference between Ax [the x position of point A] and Ox as a side and the difference between Ay and Oy as another. We can get the hypotenuse with the pythagorean theorem. Since cos(α) [α is the angle] is equal to adjacent ÷ hypotenuse [where adjacent is the side next to the angle α] then we an get α by acos(adjacent ÷ hypotenuse). When summing vectors all you got to do is make two triangles, add their sides [(Ax-Ox) + (Bx-Ox) & (Ay-Oy) + (By-Oy)], and get the new hypotenuse.
