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Algebra Difficulty 4.5 AIME Prove it Slovenia

Prove that the inequality x2+y2+12(xyx+y)x^2 + y^2 + 1 \ge 2(xy - x + y) holds for any two real xx and yy. When does the equality hold?

Solution

Rewrite the inequality x2+y2+12(xyx+y)x^2 + y^2 + 1 \ge 2(xy - x + y) as x22xy+y2+2x2y+10x^2 - 2xy + y^2 + 2x - 2y + 1 \ge 0. If we further rearrange the left-hand side into (xy)2+2(xy)+10(x - y)^2 + 2(x - y) + 1 \ge 0, we notice that it is a perfect square, and the inequality becomes ((xy)+1)20((x - y) + 1)^2 \ge 0. Hence, the inequality holds for all real xx and yy. The equality case occurs if and only if (xy)+1=0(x - y) + 1 = 0.

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