### Instruction: What is the period of $y = \cos \frac{x}{2}$?### Response: Let's use the formula $T = \frac{2\pi}{b}$.
Ok. But what is $b$?
$b$ is the coefficient of $x$ in the cosine function. So, $b = \frac{1}{2}$.
Now we can find the period. $T = \frac{2\pi}{\frac{1}{2}}$.