A = ( x2 + 4y2 + 25 - 4xy + 10x - 20y ) + ( y2 - 2y + 1 ) + 2
A = ( x - y + 5 )2 + ( y - 1 )2 + 2
A ≥ 2
Dấu "=" xảy ra ⇔ \(\left\{{}\begin{matrix}\text{x - y + 5 = 0}\\y-1=0\end{matrix}\right.\)
⇔ \(\left\{{}\begin{matrix}x=-4\\y=1\end{matrix}\right.\)