Prove that the inequality x2+y2+1≥2(xy−x+y) holds for any two real x and y. When does the equality hold?
Solution
Rewrite the inequality x2+y2+1≥2(xy−x+y) as x2−2xy+y2+2x−2y+1≥0. If we further rearrange the left-hand side into (x−y)2+2(x−y)+1≥0, we notice that it is a perfect square, and the inequality becomes ((x−y)+1)2≥0. Hence, the inequality holds for all real x and y. The equality case occurs if and only if (x−y)+1=0.
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