3 1) (x + 1)(x + 2); 3) (x + 4) (y – 4); 5) (x + a) (y a); 7) (-m – n)(m – n);

3. Умножение многочленов

1) $$(x + 1)(x + 2) = x^2 + 2x + x + 2 = x^2 + 3x + 2$$

2) $$(x + 4)(y - 4) = xy - 4x + 4y - 16$$

3) $$(x + a)(y - a) = xy - ax + ay - a^2$$

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

Ответ: 1) $$x^2 + 3x + 2$$, 2) $$xy - 4x + 4y - 16$$, 3) $$xy - ax + ay - a^2$$, 4) $$-m^2 + n^2$$