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Algebra with trinomial

Source: St Petersburg Olympiad 2014, Grade 10, P1

October 26, 2017
algebra

Problem Statement

f(x)f(x) is square polynomial and aba \neq b such that f(a)=b,f(b)=af(a)=b,f(b)=a. Prove that there is not other pair (c,d)(c,d) that f(c)=d,f(d)=cf(c)=d,f(d)=c