Combination Theorem for Continuous Functions/Real/Product Rule

Theorem
Let $X$ be one of the standard number fields $\Q, \R, \C$.

Let $f$ and $g$ be functions which are continuous on an open subset $S \subseteq X$.

Then:
 * $f g $ is continuous on $S$.

Proof
By definition of continuous:
 * $\forall c \in S: \displaystyle \lim_{x \mathop \to c} \ f \left({x}\right) = f \left({c}\right)$
 * $\forall c \in S: \displaystyle \lim_{x \mathop \to c} \ g \left({x}\right) = g \left({c}\right)$

Let $f$ and $g$ tend to the following limits:
 * $\displaystyle \lim_{x \mathop \to c} \ f \left({x}\right) = l$
 * $\displaystyle \lim_{x \mathop \to c} \ g \left({x}\right) = m$

From the Product Rule for Limits of Functions:
 * $\displaystyle \lim_{x \mathop \to c} \ \left({f \left({x}\right) g \left({x}\right)}\right) = l m$

So, by definition of continuous again, we have that $f g $ is continuous on $S$.