10) ac⁴-c⁴-ac²+c².

10) $$ac^4 - c^4 - ac^2 + c^2 = c^4(a - 1) - c^2(a-1) = (a - 1)(c^4 - c^2) = c^2(a - 1)(c^2 - 1) = c^2(a - 1)(c - 1)(c + 1)$$