Even Power is Non-Negative
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Theorem
Let $x \in \R$ be a real number.
Let $n \in \Z$ be an even integer.
Then $x^n \ge 0$.
That is, all even powers are positive.
Proof
Let $n \in \Z$ be an even integer.
Then $n = 2 k$ for some $k \in \Z$.
Thus:
- $\forall x \in \R: x^n = x^{2 k} = \paren {x^k}^2$
But from Square of Real Number is Non-Negative:
- $\forall x \in \R: \paren {x^k}^2 \ge 0$
and so there is no real number whose square is negative.
The result follows from Solution to Quadratic Equation.
$\blacksquare$
Sources
- 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 1$: Real Numbers: Exercise $\S 1.12 \ (1)$