Existence of Positive Root of Positive Real Number/Positive Exponent

Theorem
Let $x \in \R$ be a real number such that $x \ge 0$.

Let $n \in \Z$ be an integer such that $n > 0$.

Then there exists a $y \in \R: y \ge 0$ such that $y^n = x$.

Proof
Let $f$ be the real function defined on the unbounded closed interval $\left[{0 \,.\,.\, \to}\right)$ defined by $f \left({y}\right) = y^n$.

Consider first the case of $n > 0$.

By Strictly Positive Integer Power Function Unbounded Above:
 * $\exists q \in \R_{>0}: f \left({q}\right) \ge x$

Since $x \ge 0$:
 * $f \left({0}\right) \le x$

By the Intermediate Value Theorem:
 * $\exists y \in \R: 0 \le y \le q, f \left({y}\right) = x$

Hence the result has been shown to hold for $n > 0$.