Presumably you mean the graph has 90 degree rotational symmetry about the origin. Does it help if you work out what that means in terms of the function itself? f(0)=0 f(-x)=-f(x) and so on. It sounds like something where the exact wording of the question could matter - do you have a more precise formulation, or do you know whether you should assume some form of the axiom of choice for the construction?
Assuming what you have works (you'll need to tweak a couple of inequalities, but something along those lines sounds good) you can just define f(0)=0, yes. Then prove that it satisfies the conditions and you're done.
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Does it help if you work out what that means in terms of the function itself?
f(0)=0
f(-x)=-f(x)
and so on.
It sounds like something where the exact wording of the question could matter - do you have a more precise formulation, or do you know whether you should assume some form of the axiom of choice for the construction?
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We've adjusted it like so:
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