x^2-8x+4
=x^2-8x+16-16+4
=(x-4)^2-12
x^2+y^2+xy-3y+3=0
(x^2+xy+1/4y^2)+(3/4y^2-3y+3)=0
(x+1/2y)^2+3/4(y-2)^2=0
∴{x+1/2y=0
y-2=0
解得{x=-1, y=2
x的y次方=1
(x-1)(x+1)-x(x-3)
=x^2-1-x^2+3x
=3x-1
当x=3时
原式=9-1=8
x^2-8x+4
=x^2-8x+16-16+4
=(x-4)^2-12
x^2+y^2+xy-3y+3=0
(x^2+xy+1/4y^2)+(3/4y^2-3y+3)=0
(x+1/2y)^2+3/4(y-2)^2=0
∴{x+1/2y=0
y-2=0
解得{x=-1, y=2
x的y次方=1
(x-1)(x+1)-x(x-3)
=x^2-1-x^2+3x
=3x-1
当x=3时
原式=9-1=8