# Combination Theorem for Limits of Functions/Real/Product Rule

## Theorem

Let $\R$ denote the real numbers.

Let $f$ and $g$ be real functions defined on an open subset $S \subseteq \R$, except possibly at the point $c \in S$.

Let $f$ and $g$ tend to the following limits:

$\ds \lim_{x \mathop \to c} \map f x = l$
$\ds \lim_{x \mathop \to c} \map g x = m$

Then:

$\ds \lim_{x \mathop \to c} \paren {\map f x \map g x} = l m$

## Proof

Let $\sequence {x_n}$ be a sequence of elements of $S$ such that:

$\forall n \in \N: x_n \ne c$
$\ds \lim_{n \mathop \to \infty} x_n = c$
$\ds \lim_{n \mathop \to \infty} \map f {x_n} = l$
$\ds \lim_{n \mathop \to \infty} \map g {x_n} = m$
$\ds \lim_{n \mathop \to \infty} \paren {\map f {x_n} \map g {x_n} } = l m$

Applying Limit of Real Function by Convergent Sequences again, we get:

$\ds \lim_{x \mathop \to c} \paren {\map f x \map g x} = l m$

$\blacksquare$