a) x4y4 + 4
= (xy)4 + 4x2y2 + 22 - 4x2y2
= (x2y2 + 2)2 - (2xy)2
= (x2y2 - 2xy + 2)(x2y2 + 2xy + 2)
b) x4y4 + 64
= (xy)4 + 16x2y2 + 82 - 16x2y2
= (x2y2 + 8)2 - (4xy)2
= (x2y2 - 4xy + 8)(x2y2 + 4xy + 8)
c) x2 + 8x + 7
= x2 + 8x + 42 - 9
= (x + 4)2 - 32
= (x + 4 - 3)(x + 4 + 3)
= (x + 1)(x + 7)