Balanced Set in Vector Space is Symmetric

Theorem
Let $X$ be a vector space over $\R$.

Let $C \subseteq X$ be a balanced set.

Then $C$ is symmetric.

Proof
Since $C$ is balanced, we have:


 * $s C \subseteq C$ for all $s \in \C$ with $\cmod s \le 1$.

So in particular, setting $s = -1$:


 * $-C \subseteq C$

So $C$ is symmetric.