A=(x-3)(x-5)+2=x^2-5x-3x+15+2=x^2-8x+17=x^2-8x+16+1=(x-4)^2+1>0
B=x^2-5x+7=x^2-5/2*2x+(5/2)^2-(5/2)^2+7=(x-5/2)^2+3/4>0
C=x^2-xy+y^2=x^2-1/2*2xy+1/4y^2-1/4y^2+y^2=(x-1/2y)^2+3/4y^2>0
A=(x-3)(x-5)+2
=x2-8x+15+2
=x2-8x+16+1
=(x-4)2+1
vì (x-4)2 lớn hơn hoặc = 0 nên (x-4)2+1 dương