Ta có :5x2+2y2+4xy−2x+4y+2015
=(4x2+4xy+y2)+(x2−2x+1)+(y2+4y+4)+2010
=(2x+y)2+(x−1)2+(y+2)2+2010
Vì (2x+y)2≥0 ; (x−1)2≥0 ; (2+y)2≥0
=>(2x+y)2+(x−1)2+(y+2)2≥0
=>(2x+y)2+(x−1)2+(y+2)2+2010≥2010
Hay 5x2+2y2+4xy−2x+4y+2015≥2010
Dấu "=" xảy ra khi : x = 1 ; y = -2
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