Maths Olympiad Prep

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Algebra Difficulty 4.4 AIME Prove it Philippines

Problem:
Find the domain of the function f(x)=1[x2x2]f(x)=\frac{1}{\left[\left|x^{2}-x-2\right|\right]}.

Solution

Solution:
xx must not satisfy 0x2x2<1x0 \leq x^{2}-x-2<1 \Rightarrow x must not satisfy x2x2=(x2)(x+1)0x(,1][2,+)x^{2}-x-2=(x-2)(x+1) \geq 0 \Leftrightarrow x \in(-\infty,-1] \cup[2,+\infty) AND must not satisfy x2x21x2x3<0x(1132,1+132)x^{2}-x-2 \leq 1 \Leftrightarrow x^{2}-x-3<0 \Leftrightarrow x \in\left(\frac{1-\sqrt{13}}{2}, \frac{1+\sqrt{13}}{2}\right). Thus, x(,1132](1,2)[1+132,+)x \in\left(-\infty, \frac{1-\sqrt{13}}{2}\right] \cup(-1,2) \cup\left[\frac{1+\sqrt{13}}{2},+\infty\right).

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