d) $a^{3}+b^{3}+c^{3} = (a + b)^3 - 3ab(a + b) + c^3 = - c^3 + c^3 - 3ab(- c) = 3abc$
c) $2x^{4}-7x^{3}-2x^{2}+13x+6\\
= 2x^4 + x^3 - 8x^3 - 4x^2 + 2x^2 + x + 12x + 6\\
= x^3(2x + 1) - 4x^2(2x + 1) + x(2x + 1) + 6(2x + 1)\\
= (2x + 1)(x^3 - 4x^2 + x + 6)\\
= (2x + 1)(x + 1)(x - 3)(x - 2)$
Xét...