give the value of f(x)=cos(x/2) at 0

We save the definition: $f\left(x\right)=\cos \left(\frac{x}{2}\right)$

Using stored definitions, replace $f\left(0\right)$ with $\cos \left(\frac{0}{2}\right)$

Let's calculate: $\cos \left(\frac{0}{2}\right)$

$\cos \left(0\right)$

ⓘ The cosine is the x-coordinate (horizontal axis) of the point reached on the unit circle after a rotation of $0$ radians from the horizontal axis pointing to the right, this is a remarkable value: $\cos \left(0\right)$ = $1$

▶Result:   $1$