# Is it constant or not?

Number Theory Level 3

Let $$X$$ be the set of all positive integers greater than or equal to 8 and let $$f: X \to X$$ be a function such that $$f(x+y)=f(xy)$$ for all $$x$$ and $$y$$ greater than or equal to 4. If $$f(8)=9$$. Find $$f(9)$$.

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