Đáp án:
`A=x^2+2y^2-2xy-2x+8y-1`
`=x^2-2xy+y^2+y^2-2(x-y)+6y-1`
`=(x-y)^2-2(x-y)+1+y^2+6y+9-11`
`=(x-y-1)^2+(y+5)^2-11>=-11`
Dấu "=" xảy ra khi \(\begin{cases}x-y-1=0\\y+5=0\\\end{cases}\)`<=>` \(\begin{cases}y=-5\\x=-4\\\end{cases}\)
`B=x^2-x-7`
`=x^2-x+1/4-1/4-7`
`=(x-1/2)^2-29/4>=-29/4`
Dấu "=" xảy ra khi `x-1/2=0<=>x=1/2`