Hello.

I have just encountered this problem. I want to find a cosine function with the graph represented on the picture below.

I have found the following:

$\displaystyle \rm{A} \times \cos(cx + \phi) + d$

$\displaystyle 2 \times \cos(\frac \pi 2x + \phi)$

$\displaystyle d=0$

But I am unable to find $\displaystyle \phi$. So then my question would be: How do I find it, please?

In advance, thank you for spending time and efforts on my problems :]