Get Angle Between Two Vectors On A Circle

Question

I've a P1,P2 and P3 vectors and I need to angle. It has to positive. For instance α = 30, θ=270

Have can I calculate P1->P2;α or p1->P3;θ ?

Answer 1

Related Wikipedia article: Dot product

By calculating the dot product of the two vectors and then dividing by the product of their magnitudes, you get cos(angle). Using acos, you can then extract the angle.