Maths Olympiad Prep

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, 2022

Algebra Difficulty 5.0 AIME Prove it United States

Problem:

Two linear functions f(x)f(x) and g(x)g(x) satisfy the properties that for all xx,
- f(x)+g(x)=2f(x) + g(x) = 2
- f(f(x))=g(g(x))f(f(x)) = g(g(x))
and f(0)=2022f(0) = 2022. Compute f(1)f(1).

Solutions — 2

Solution 1

Solution:

Firstly, f(x)f(x) and g(x)g(x) must intersect - otherwise, f(x)=g(x)=1f(x) = g(x) = 1, which can't be true.

Secondly, suppose they intersect at aa, so that f(a)=g(a)=cf(a) = g(a) = c. Then f(c)=g(c)f(c)=g(c)=1f(c) = g(c) \Longrightarrow f(c) = g(c) = 1. But then, a=ca = c, and c=1c = 1. So f(1)=1f(1) = 1, and we're done.

Solution 2

Solution:

We will solve the problem manually, setting f(x)=ax+2022f(x) = a x + 2022 and g(x)=ax2020g(x) = -a x - 2020. Then f(f(x))=a2x+2022a+2022f(f(x)) = a^{2} x + 2022 a + 2022, while g(g(x))=a2x+2020a2020g(g(x)) = a^{2} x + 2020 a - 2020.

Then, a=2021a = -2021, so then f(1)=1f(1) = 1.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.