Why is f(n) of class O(g(n)) in this graph?
I think you're confused about the notion of upper bounds for a function. We're not trying to compute an upper bound for the function&nbsp;$f$ in the sense of saying that, e.g., $f(n) \leq 45$ for all&nbsp;$n$ (or all&nbsp;$n$ greater than some&nbsp;$n_0$). Rather, we're trying to say that, for all $n&gt;n_0$, the value (number) $c\,g(n)$ is an upper bound for the value $f(n)$. The graph indicates this informally (since it only shows what happens for some unstated but limited range of values of&nbsp;$n$).