In other words, if $f(x_1) = f(x_2)$, then $x_1$ must equal $x_2$.
For the function $f(x) = \sqrt{x}$, let's assume $f(x_1) = f(x_2)$. This means that $\sqrt{x_1} = \sqrt{x_2}$. Squaring both sides, we get $x_1 = x_2$. Since this is true, the function is
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