2bx^2-8bxy+8by^2-8b=2b(x^2-4xy+4y^2)-8b=2b(x-2y)^2-8b=2b*(-2)^2-8b=0
2bx^2-8bxy+8by^2-8b=2b(x^2-4xy+4y^2-4)=2b((x-2y)^2-4)=2b((-2)^2-4)=0
2bx^2-8bxy+8by^2-8b
=2b(x^2-4xy+4y^2-4)
=2b[(x-2y)^2-4]
=2b[(-2)^2-4]
=0
2bx^2-8bxy+8by^2-8b
=2b(x^2-4xy+4y)-8b
=2b(x-2y)^2-8b
=2b^4-8b
=0