b) x^2+2xy+y^2= (x+y)^2
x^2+2xy+y^2- (x+y)^2= (x+y)^2-(x+y)^2=0
c) x^2-2xy+y^2= (x-y)^2
x^2-2xy+y^2- (x-y)^2= (x-y)^2-(x-y)^2=0
