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Problem 1145

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Algebra Difficulty 5.1 Prove it Slovenian Mathematical Olympiad · Slovenia · 2008

Find all real numbers xx for which the inequality
2xx82008 |||2 - x| - x| - 8| \le 2008
holds.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

We consider two cases depending on the sign of 2x2-x.

If x2x \ge 2, then we have 2x=(2x)|2-x| = -(2-x) and the inequality can be rewritten as x2x82008||x-2-x|-8| \le 2008 or, equivalently, 620086 \le 2008. So for all real numbers x2x \ge 2 the inequality holds.

Now, let x<2x < 2. In this case we have
22x82008. ||2 - 2x| - 8| \le 2008.
Let us consider two further cases depending on the sign of 22x2-2x.

If x1x \ge 1, we have 22x=(22x)|2-2x| = -(2-2x) and so 2x102008|2x-10| \le 2008. Obviously, this inequality holds for 1x<21 \le x < 2.

Finally, only the case x<1x < 1 remains. The condition implies 2x+282008|-2x+2-8| \le 2008 or 2x+62008|2x+6| \le 2008. This is equivalent to
20082x+62008. -2008 \le 2x + 6 \le 2008.
Since x<1x < 1 we have 2x+6<2+6=82x + 6 < 2 + 6 = 8, so the right inequality holds. The left inequality implies 20142x-2014 \le 2x or 1007x-1007 \le x. From this we derive the condition 1007x<1-1007 \le x < 1.

Hence, the inequality holds for all x1007x \ge -1007.

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