Normal
b) PT = (x^2 - 2x + 1) + (y^2 - 2y+1) +(xy-x-y+1)= (x-1)^2 + (y-1)^2 + (x-1)(y-1)PT =0 <=> x=y=0
b) PT = (x^2 - 2x + 1) + (y^2 - 2y+1) +(xy-x-y+1)
= (x-1)^2 + (y-1)^2 + (x-1)(y-1)
PT =0 <=> x=y=0
