Function or not?

[Cohiba]

I'v got problems like this:

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{(x,y) : y^2 = x }

and I have to state wether it is a function or not.

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so, let y = 2
(2)^2 = 4

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let y = -2
(-2) ^2 = 4

= not a function because (4,2), (4,-2).

but i encountered this problem:

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{(x,y) :|x + y| = 4 }

and I have no idea how to know if it is a function or not and demonstrate it like i did with the other one. Thanks.

[MXP]

Let y = 0.
What value(s) can x have? If there is more than one value that x can have then y(x) is not a function.

[t i l e x]

You are wrong Colin. By definition, a function is an equation where there aren't 2 y-values for the same x.

[MXP]

Err, I was backwards. Let me rephrase:

Let x = 0.
What value(s) can y have? If there is more than one value that y can have then y(x) is not a function.

[Cohiba]

let x = 0
|y + x| = 4
|y| = 4
y = 4 or y = -4

(0,4), (0,-4). so, not a function.

Thanks guys.