`B = x^2 - 4x + 3y^2 + 4y-15`
`= (x^2 - 4x+4) + 3(y^2 + 4/3y + 4/9) - 61/3`
`= (x-2)^2 + 3(y+2/3)^2 - 61/3`
Do: `(x - 2)^2 ≥ 0` `∀` x; `3(y+2/3)^2 ≥ 0` `∀` y
`=> (x-2)^2 + 3(y_2/3)^2 >= 0`
`=> B = (x-2)^2 + 3(y+2/3)^2 - 61/3 ≥ -61/3`
Dấu "=" xảy ra khi $\begin{cases} x = 2\\ y = \dfrac{-2}{3} \end{cases}$
Vậy GTNN của `B = -61/3 <=> x = 2; y = -2/3`