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Slovenija 2008

Slovenia 2008 algebra

Problem

Find all real numbers for which the inequality holds.
Solution
We consider two cases depending on the sign of .

If , then we have and the inequality can be rewritten as or, equivalently, . So for all real numbers the inequality holds.

Now, let . In this case we have Let us consider two further cases depending on the sign of .

If , we have and so . Obviously, this inequality holds for .

Finally, only the case remains. The condition implies or . This is equivalent to Since we have , so the right inequality holds. The left inequality implies or . From this we derive the condition .

Hence, the inequality holds for all .
Final answer
x ≥ -1007

Techniques

Linear and quadratic inequalities