Combination Theorem for Continuous Functions/Real/Multiple Rule

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

Let $f$ be a function which is continuous on an open subset $S \subseteq X$.

Let $\lambda \in X$ be an arbitrary number in $X$.

Then:
 * $\lambda f$ is continuous on $S$.

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

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

From the Multiple Rule for Limits of Functions, we have that:
 * $\displaystyle \lim_{x \mathop \to c} \ \left({\lambda f \left({x}\right)}\right) = \lambda l$

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