If $f$ is continuous on a closed interval $[a,b]$, then $f$ attains both an
absolute maximum value and an absolute minimum value at some
numbers in $[a,b]$.
This theorem tells us that we don't have to worry about whether absolute maxima or minima
occur, just about where they are. This may seem obvious, but the
theorem does not apply
(meaning that it may not have
an absolute maximum or minimum) when the function is discontinuous or when the interval isn't closed; for example:
DO: Try to find the
maximum value of $1/x$ on $[-1,1]$.
DO: Try to find the
maximum value of $f(x)=1/x$ on $(0,1)$.