Zero is both Positive and Negative
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Theorem
The number $0$ (zero) is the only (real) number which is both:
and
Proof
Let $x$ be a real number which is both positive and negative.
Thus:
- $x \in \set {x \in \R: x \ge 0}$
and:
- $x \in \set {x \in \R: x \le 0}$
and so:
- $0 \le x \le 0$
from which:
- $x = 0$
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $1$: Algebraic Structures: $\S 1$: The Language of Set Theory