X + y = z (constant) - what variation is that?

[moonman239]

I have the equation x + y = z. Z is a constant. What type of variation is expressed here?

An example of direct variation is x*z = y.
An example of inverse variation of x * y = z.
In both examples, z is a constant.

So what's the answer?

 

[AnTiFreeze3]

Direct and indirect variations are essentially proportions, right? How are you using addition?

 

[HallsofIvy]

There is no "proportion" here.