Let $f : \mathbb{R} → \mathbb{R}$ be continuous, with $f(x)f(f(x)) = 1$ for all $x ∈ \mathbb{R}$. If $f(1000) = 999$, find $f(500)$.
Hint 1: What can you say about $f(y)$ if $y$ is in the image of $f$?
Hint 2: The intermediate value theorem will be helpful. Note that $999$ is in the image of $f$.
Hint Use Intermediate Value Theorem between $\frac{1}{999}$ and $999$.
$500$ belongs in this interval.