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Bạn tham khảo bài.
`C=x^2 -2x+y^2 +4y+21`
`->C=(x^2 - 2 . x . 1 +1^2) + (y^2 + 2 . y . 2 + 2^2) + 16`
`->C=(x-1)^2 + (y+2)^2+ 16`
Vì `(x-1)^2 ≥0∀x, (y+2)^2 ≥0∀y`
`->(x-1)^2 + (y+2)^2 + 16 ≥ 16∀x,y`
`->C ≥ 16 ∀x,y`
Dấu "`=`" xảy ra khi :
`(x-1)^2=0,(y+2)^2=0`
`↔x-1=0,y+2=0`
`↔x=1,y=-2`
Vậy `min C=16↔x=1,y=-2`
