They're really two different types of equal signs.

f(x,y) = x+y might be better written as f(x,y) := x+y where := means "is defined as". Then f(x,y) = 0 is an equation that expands to x+y = 0, or in familiar intro algebra form, y=-x.

g(x,y) := 0 really is a flat plane.

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wholinator2 13 hours ago | parent [-]

I'd seen the := in programming for years but always thought it was basically just =. Thank you for your explanation!

	▲	roywiggins 12 hours ago \| parent [-]
		I will say that in programming it's commonly used as assignment, which isn't quite the same thing as definition. golang uses it to declare variables so that's pretty close