Answer
Fill the blank with
... $\cos y$ ...
Work Step by Step
When we restrict the domain of $\cos x$ to $[0,\pi]$, it becomes one-to-one and has an inverse.
For inverse functions,
$f(f^{-1}(y))=y$ and
$f^{-1}(f(x))=x.$
So, with $\cos^{-1}x$ being the inverse of $\cos x$ (on the restricted domain),
$\cos y=\cos[\cos^{-1}(x)]=x.$
Fill the blank with
... $\cos y$ ...
