4-4a2+8ab-4b2
(a2+b2-1)2-4a2b2
x4+x2y2+y4
x6-x5+x4+x3+1
a2-8ab+16b2+6a-24b+9
(x+2y)(3x-7y)2-4(x+2y)(x+y)2
(a+2b)3+(a-2b)3