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Is y2=x not a function?

The 2 is an exponent 

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Kurt T. | Math Tutoring and Test PrepMath Tutoring and Test Prep
4.9 4.9 (124 lesson ratings) (124)
It is not, because the definition of a function includes the requirement that no x will have more than one corresponding y.  The parabola you named has its vertex at (0,0) and opens to the right, so every x value > 0 will have two corresponding y values, one positive and one negative.  For example, if x = 4, y = 2 and -2.  Remember the vertical line test; if you can draw a vertical line anywhere that crosses more than one point on the graph, it is not a function.