[tex]A=-(x^{2}-2xy+4y^{2}-2x-10y+3)
=-(x^{2}-2x(y+1)+(y+1)^{2}+3(y^{2}-4y+4)-9)
=-((x-y-1)^{2}+3.(y-2)^{2}-9)
=-(x-y-1)^{2}-3(y-2)^{2}+9\leq 9[/tex]
Max A =9 khi x=3 ; y=2
[tex]A=-(x^{2}-2xy+4y^{2}-2x-10y+3)
=-(x^{2}-2x(y+1)+(y+1)^{2}+3(y^{2}-4y+4)-9)
=-((x-y-1)^{2}+3.(y-2)^{2}-9)
=-(x-y-1)^{2}-3(y-2)^{2}+9\leq 9[/tex]
Max A =9 khi x=3 ; y=2