Solution
= Solution
For every $x\in\mathbb R$, the definition of <function composition> gives
$$
\bigl(h\circ(g\circ f)\bigr)(x)
=h\bigl((g\circ f)(x)\bigr)
=h(g(f(x)))
=\bigl((h\circ g)\circ f\bigr)(x).
$$
The <functions> agree at every argument, so <function extensionality> proves the <associative property> of <function composition>.