Question 336047
by definition, y=f(x) can be a function if for each x value there can be only one y. 
The vertical line test, is that if you plot y and you can draw a vertical line and touch more than one y point, it is not a function.
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if you solve {{{4*x=y^2}}} for y, you get  {{{y=-2*sqrt(x)}}} and {{{y=2*sqrt(x)}}}, which violates the definition of a function, more than one y for each x.