How to calculate the new intersection on the x-axis after rotation of a rectangle? I've been trying to calculate the new intersection on the x-axis after rotation of any given rectangle.  The rectangle's center is the point $(0,0)$.
What do I know:


*

*length of B (that is half of the width of the given rectangle)  

*angle of a (that is the rotation of the rectangle)  


What do I want to know:
length of A (or value of point c on the x-axis).  

Hint: Try to divide the cases. Referring to your image, after the rotation of the angle $a$ the vertex on the left side of the rectangle pass or not pass the x-axis? 
Suppose now that your rectangle has one side of lenght 2B, and the other one "large", so the vertex on the left side doesn't pass the x-axis. Then using Pythagoras you get $A=\sqrt{B^2 + B^2 sen^2(a)}$. 
What about the other case?
