\(B=x^2+4y^2+1+4xy-2x-4y+3\left(y^2+2.\frac{1}{3}y+\frac{1}{9}\right)-\frac{34}{3}\)
\(B=\left(x+2y-1\right)^2+3\left(y+\frac{1}{3}\right)^2-\frac{34}{3}\ge-\frac{34}{3}\)
\(B_{min}=-\frac{34}{3}\) khi \(\left\{{}\begin{matrix}y=-\frac{1}{3}\\x=\frac{5}{3}\end{matrix}\right.\)