Ononot — A function f: \mathbb{R} \to \mathbb{R} is defined as f(x) =

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Functionshard

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A function $f: \mathbb{R} \to \mathbb{R}$ is defined as $f(x) = ax^2 + bx + c$ . If $f(f(x)) = x$ for all $x$ , which of the following must be true?