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UKMT Senior Mathematical Challenge, 2018

Algebra Difficulty 3.0 AMC 10/12 Find the answer United Kingdom

For particular real numbers aa and bb, the function ff is defined by f(x)=ax+bf(x)=ax+b, and is such that f(f(f(x)))=27x−52f(f(f(x)))=27x-52. Which of the following formulas defines the function gg such that, for all values of xx, g(f(x))=xg(f(x))=x?

(A) 13x−4\frac{1}{3}x-4 (B) 13x+43\frac{1}{3}x+\frac{4}{3} (C) 4x−34x-3 (D) 13x−43\frac{1}{3}x-\frac{4}{3} (E) 3x−43x-4

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