4. 4 1) (x² + b) (x² + 2b); 3) (2m + n²) (2n – m²); 5) (n + 2m)(n – 2m);

4. Умножение многочленов

1) $$(x^2 + b)(x^2 + 2b) = x^4 + 2bx^2 + bx^2 + 2b^2 = x^4 + 3bx^2 + 2b^2$$

2) $$(2m + n^2)(2n - m^2) = 4mn - 2m^3 + 2n^2 - m^2n^2$$

3) $$(n + 2m)(n - 2m) = n^2 - 2mn + 2mn - 4m^2 = n^2 - 4m^2$$

Ответ: 1) $$x^4 + 3bx^2 + 2b^2$$, 2) $$4mn - 2m^3 + 2n^2 - m^2n^2$$, 3) $$n^2 - 4m^2$$