![\textbf{You have to}](https://img.qammunity.org/2019/formulas/mathematics/high-school/ykrkz0wtj48j8hrajzgl0ofmvu5qkrlgll.png)
restrict the domains of quadratic functions and absolute value functions, because these functions are
![\textbf{many-to-one}](https://img.qammunity.org/2019/formulas/mathematics/high-school/99ahw2nivk91e8bmp4yunmrwf2ekxu3tyf.png)
functions. For instance, the quadratic function f(x) = x^2 pairs both −2 and 2 with 4, and the absolute value function f(x) = |x| pairs both −2 and 2 with 2.
Linear functions (excluding constant functions) and exponential functions are
![\textbf{one-to-one}](https://img.qammunity.org/2019/formulas/mathematics/high-school/nvxpblc87gij0wr8766h8a1ddw851xxz74.png)
functions, so their domains
![\textbf{do not need}](https://img.qammunity.org/2019/formulas/mathematics/high-school/3ru0tu1n26qtas1gz2vvdx6j3syo18fpvc.png)
to be restricted.
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An absolute value function, without domain restriction, has an inverse that is NOT a function.
In order to guarantee that the inverse must also be a function, we need to restrict the domain of the absolute value function to make it a one-to-one function.