Ta có C = x2 - 4x + y2 - y + 5
= \(\left(x^2-4x+4\right)+\left(y^2-y+\frac{1}{4}\right)+\frac{3}{4}\)
= \(\left(x-2\right)^2+\left(y-\frac{1}{2}\right)^2+\frac{3}{4}\ge\frac{3}{4}\forall x\)
=> Min C = 3/4
Dấu "=" xảy ra <=> \(\hept{\begin{cases}x-2=0\\y-\frac{1}{2}=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=2\\y=\frac{1}{2}\end{cases}}\)
Vậy Min C = 3/4 <=> x = 2 ; y = 1/2
C = ( x2 - 4x + 4 ) + ( y2 - y + 1/4 ) + 3/4
= ( x - 2 )2 + ( y - 1/2 )2 + 3/4 ≥ 3/4 ∀ x.y
Dấu "=" xảy ra <=> x = 2 ; y = 1/2 . Vậy MinC = 3/4
