+972 Let A1 A2 A3 ... A15 be a regular polygon. A rotation centered at $A_4$ with an angle of $\alpha$ takes $A_3$ to $A_5$. Given that $\alpha < 180^\circ$, find $\alpha,$ in degrees.
+9675 
This is the diagram. You can try to rotate A_3 around A_4 in your mind. Notice the angle in question is just an interior angle of the regular polygon.
We use the formula for an interior angle of a regular n-gon: α=(n−2)×180∘n. Here, n is the number of sides, which is 15 in this case.
We get α=(15−2)×180∘15=156∘.
