# Combination Theorem for Continuous Functions/Sum Rule

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## 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, we have that

- $\forall c \in S: \displaystyle \lim_{x \to c} \ f \left({x}\right) = f \left({c}\right)$
- $\forall c \in S: \displaystyle \lim_{x \to c} \ g \left({x}\right) = g \left({c}\right)$

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

- $\displaystyle \lim_{x \to c} \ f \left({x}\right) = l$
- $\displaystyle \lim_{x \to c} \ g \left({x}\right) = m$

From the Sum Rule for Limits of Functions, we have that:

- $\displaystyle \lim_{x \to c} \ \left({f \left({x}\right) + g \left({x}\right)}\right) = l + m$

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

$\blacksquare$