How to calculate obtuse angle between two vectors?

Question

How to calculate obtuse angle between two vectors if both vectors and x axis is given. First vector can be taken as x axis. we can get acute by dot product and acos.

Answer 1

Before applying acos, check if the dot product is negative. If negative, the angle is obtuse :)

Further, as acos has a range of 0 to pi, you will do fine as long as you do not want reflex angles (>pi)