Angle between 3 vertices

Question

For example, GetAngle((0,0),(100,0),(100,100)) = 90. How could I find the angle between 3 2D Points.

Answer 1

Given points A, B, and C, you want the angle between AB and AC? First compute the vectors AB and AC -- it's just the coordinates of B minus coordinates of A and likewise for AC. Take the dot product of the two vectors. This is just the product of the x coordinates plus the product of the y coordinates of the vectors. Divide this number by the length of AB, and again by the length of AC. This result is the cosine of the angle between the two vectors, so take the arccos() and you have it.