Combination Theorem for Continuous Mappings/Metric Space/Product Rule

Theorem
Let $M = \struct {A, d}$ be a metric space.

Let $\R$ denote the real numbers.

Let $f: M \to \R$ and $g: M \to \R$ be real-valued functions from $M$ to $\R$ which are continuous on $M$.

Then:
 * $f g$ is ‎continuous on $M$.

Proof
By definition of ‎continuous:
 * $\forall a \in M: \ds \lim_{x \mathop \to a} \map f x = \map f a$
 * $\forall a \in M: \ds \lim_{x \mathop \to a} \map g x = \map g a$

Let $f$ and $g$ tend to the following limits:
 * $\ds \lim_{x \mathop \to a} \map f x = l$
 * $\ds \lim_{x \mathop \to a} \map g x = m$

From the Product Rule for Limits of Real Functions, we have that:
 * $\ds \lim_{x \mathop \to a} \paren {\map f x \map g x} = l m$

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