Function or relation: $y^4 = 8x^2$
The question under debate: Is $y^4=8x^2$ a function or a relation?
We disagree. I say, it's a relation because when I substitute in points
I get two outputs for most inputs, and thus it's not a function (fails the vertical line test). I also used Wolfram Alpha and Desmos to check the graph, and both of those yield vertical-line-failing horizontal parabolas...
which fail the vertical line test.
Another person says "...because the original problem did not include a root... it was an exponent - we quad-rooted the other side to solve for y only... but that wasn't the original question/problem. So the way the original problem is set up, there would be no negative answers... " which means each x has one y, it passes the vertical line test, and it’s a function.
I'll admit, I don't quite understand his reasoning, but there's a lot I don't understand, like how the square root function yields only a positive result but $x^2=9$ yields $+3$ and $-3$, so I can't argue convincingly.
Is there some way to rule either in/out? The answer given by the system indicates my answer is correct, that it's a relation and not a function, but for a host of reasons I won't describe, we don't fully trust that system.
Thank you for any light you can shed that will resolve our debate!
$\endgroup$ 52 Answers
$\begingroup$You are definitely correct. You can regard the equation $y^4 = 8x^2$ as the relation $$xRy \iff y^4 = 8x^2,$$ and $R$ is a function of $x$ if and only if, for each $x$, there is at most one $y$ such that $xRy$. But that is clearly false in this case. For example, let $x=2$; then $y^4 = 32$ so that $y = \pm\sqrt[4]{32} = \pm 2\sqrt[4]{2}$ (as well as some complex values). Thus this relation does not define a function of $x$.
$\endgroup$ $\begingroup$$y^4=8x^2$ is an equation, not a function. Moreover, this equation does not implicitely define $y$ as a function of $x$ due to the fact that more $y$'s correspond to one $x$. Also, $x$ is not a function of $y$.
On the other hand, you can consider a function $f(x,y)=y^4-8x^2$ of two variables and the domain of satisfiability of your equation equals the zero set of $f$. That is, your equation can be reformulated to $f(x,y)=0$ where $f$ is a function.
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