\(B=\left(x^2+\frac{y^2}{4}+\frac{25}{4}+xy-5x-\frac{5}{2}y\right)+\left(\frac{3}{4}y^2-\frac{5}{2}y+\frac{25}{12}\right)+\frac{35}{3}\)
\(B=\left(x+\frac{y}{2}-\frac{5}{2}\right)^2+\frac{3}{4}\left(y-\frac{5}{3}\right)^2+\frac{35}{3}\ge\frac{35}{3}\)
\(B_{min}=\frac{35}{3}\) khi \(x=y=\frac{5}{3}\)