A = x2 + 2y2 - 2xy + 2x - 2y + 1
= x2 - 2xy + y2 + 2 ( x - y ) + 1 + y2
= ( x - y )2 + 2 ( x - y ) + 1 + y2
= ( x - y + 1 )2 + y2 ≥ 0
Dấu = xảy ra khi :
\(\left\{{}\begin{matrix}x-y+1=0\\y=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-1\\y=0\end{matrix}\right.\)
B = x2 + 2y2 - 2xy + 2x - 10y
= x2 - 2xy + y2 + 2x - 2y + 1 + y2 - 8x + 16 - 17
= ( x - y )2 + 2 ( x - y ) + 1 + ( y - 4 )2 - 17
= ( x - y + 1 )2 + ( y - 4 )2 - 17 ≥ - 17
Dấu = xảy ra khi :
\(\left\{{}\begin{matrix}x-y+1=0\\y-4=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=4\end{matrix}\right.\)
