How to solve this functional equation: $f(1-f(x))=1-x^{9}, f(1)=0$

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I have managed to guess one solution of this function : $f(x)=1-x^{3}$, but I have no idea how to prove it unique, or get other solutions.

If this is not solvable, how can you prove this function have negative derivative at interval $(0,1)$?

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A hint:

Consider the function $g(x):=1-f(x)$.