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Ukrainian National Mathematical Olympiad, 4th Round

Algebra Difficulty 6.0 AIME, harder Prove it Ukraine

max⁡{∣x∣−1,x2−1}≤min⁡{x2−1,1−x}, \max\{|x|-1, x^2-1\} \leq \min\{x^2-1, 1-x\},
where max⁡{a,b}={a,if a≥bb,if a<b\max\{a,b\} = \begin{cases} a, & \text{if } a \ge b \\ b, & \text{if } a < b \end{cases}, and min⁡{a,b}={b,if a≥ba,if a<b\min\{a,b\} = \begin{cases} b, & \text{if } a \ge b \\ a, & \text{if } a < b \end{cases}.

Solution

It is clear that
max⁡{∣x∣−1,x2−1}≥x2−1≥min⁡{x2−1,1−x}, \max\{|x|-1, x^2-1\} \geq x^2-1 \geq \min\{x^2-1, 1-x\},
so the inequality from the problem condition can be satisfied only if
max⁡{∣x∣−1,x2−1}=x2−1=min⁡{x2−1,1−x}, \max\{|x|-1, x^2-1\} = x^2-1 = \min\{x^2-1, 1-x\},
and this, in turn, implies that
∣x∣−1≤x2−1≤1−∣x∣. |x|-1 \leq x^2-1 \leq 1-|x|.
Therefore, 1−∣x∣≥∣x∣−11-|x| \geq |x|-1, which means that ∣x∣≤1|x| \leq 1. Also we should have that ∣x∣≤∣x∣2=∣x∣⋅∣x∣|x| \leq |x|^2 = |x| \cdot |x|, and since we already know that ∣x∣≤1|x| \leq 1, this can happen only if ∣x∣=1|x|=1 or ∣x∣=0|x|=0, so x=1,x=0x=1, x=0 or x=−1x=-1.

It is also easy to solve this problem using graphs (fig. 35). Here the graph of the left-hand side is drawn with dash-and-dash line, and the graph of the right-hand side – with dash-and-dot line. They intersect exactly at the specified values of xx.

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