## Precalculus (6th Edition) Blitzer

False. Generally, parabolas that define $y$ as a function of $x$ open up or down. If a parabola opens left or right, it is generally defined with $x$ as a function of $y$. Note that one can still write such a function with $y$ as a function of $x$ with a plus-minus sign, e.g. $y=\pm\sqrt x$ But this can be seen as two combined functions rather than one.