[tex]A=(x^{2}+y^{2}+1+2xy+2x+2y)-(5x+5y+5)+8=(x+y+1)^{2}-5(x+y+1)+7=(x+y+1).(x+y-4)+7=(x+y)^{2}-3.(x+y)-4+7=(x+y)^{2}-2.(x+y).\frac{3}{2}+\frac{9}{4}+\frac{19}{4}=(x+y-\frac{3}{2})^{2}+\frac{19}{4}\geq \frac{19}{4}> 0[/tex]
Không tồn tai x,y thoả mãn
[tex]A=(x^{2}+y^{2}+[COLOR=#ff0000][B]1[/B][/COLOR]+2xy+2x+2y)-(5x+5y+[COLOR=#ff0000][B]5[/B][/COLOR])+[COLOR=#ff0000][B]8[/B][/COLOR]=(x+y+1)^{2}-5(x+y+1)+8=(x+y+1).(x+y-4)+8=(x+y)^{2}-3.(x+y)-4+8=(x+y)^{2}-2.(x+y).\frac{3}{2}+\frac{9}{4}+\frac{23}{4}=(x+y-\frac{3}{2})^{2}+\frac{23}{4}\geq \frac{23}{4}> 0[/tex]
Không tồn tai x,y thoả mãn