c) x^4 – y^4 + x^3y – xy^3.
c) x4 – y4 + x3y – xy3.
c) x4 – y4 + x3y – xy3 = (x4 + x3y) – (y4 + xy3)
= x3(x + y) – y3(x + y) = (x + y)(x3 – y3)
= (x + y)(x – y)(x2 + xy + y2).