The range of the inverse cosine function, $\cos^{-1} x$, is $[0, \pi]$. This means that $0 \leq \cos^{-1} x \leq \pi$.
Given that $2 \cos^{-1} x = y$, we need to find the range of $y$. Since $0 \leq \cos^{-1} x \leq \pi$, we can multiply the inequality by 2: $$2(0) \leq 2 \cos^{-1} x \leq 2(\pi)$$ $$0 \leq y \leq 2\pi$$
Final Answer: $0 \leq y \leq 2\pi$
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