Can anyone prove the angle $\theta$ (smaller angle) between diagonals of a parallelogram is given by the equation
$$\cos(\theta)=\frac{a-c}{a+c}$$
where $2a$ is the base length of parallelogram; $2c$ is the horizontal distance to the edge when we draw a vertical line as seen in the picture below.
I have been trying to get this but haven't had a success. Thanks for your help in advance.