Intermediate Value Theorem/Corollary

Theorem
Let $I$ be a real interval.

Let $f: I \to \R$ be a real function which is continuous on $\left({a \,.\,.\, b}\right)$.

Let $a, b \in I$ such that $\left({a \,.\,.\, b}\right)$ is an open interval. Let $0 \in \R$ lie between $f \left({a}\right)$ and $f \left({b}\right)$.

That is, either:
 * $f \left({a}\right) < 0 < f \left({b}\right)$

or:
 * $f \left({b}\right) < 0 < f \left({a}\right)$

Then $f$ has a root in $\left({a \,.\,.\, b}\right)$.

Proof
Follows directly from the Intermediate Value Theorem and from the definition of root.