# Combination Theorem for Limits of Functions/Combined Sum Rule

## Theorem

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

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

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

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

Let $\lambda, \mu \in X$ be arbitrary numbers in $X$.

Then:

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

## Proof

Let $\left \langle {x_n} \right \rangle$ be any sequence of points of $S$ such that:

- $\forall n \in \N^*: x_n \ne c$
- $\displaystyle \lim_{n \mathop \to \infty} x_n = c$

By Limit of Function by Convergent Sequences:

- $\displaystyle \lim_{n \mathop \to \infty} f \left({x_n}\right) = l$
- $\displaystyle \lim_{n \mathop \to \infty} g \left({x_n}\right) = m$

By the Combined Sum Rule for Sequences:

- $\displaystyle \lim_{n \mathop \to \infty} \left({\lambda f \left({x_n}\right) + \mu g \left({x_n}\right)}\right) = \lambda l + \mu m$

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

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

$\blacksquare$

## Sources

- 1977: K.G. Binmore:
*Mathematical Analysis: A Straightforward Approach*... (previous) ... (next): $\S 8.12 \ \text{(i)}$ - 1977: K.G. Binmore:
*Mathematical Analysis: A Straightforward Approach*... (previous) ... (next): Appendix: $\S 18.6$: Limits of functions