23.21. a) (x + 4)(x − 3) + (x - 5)(x + 4) = 0;

a) $$ (x + 4)(x - 3) + (x - 5)(x + 4) = 0 $$

Раскроем скобки:

$$ x^2 - 3x + 4x - 12 + x^2 + 4x - 5x - 20 = 0 $$

$$ 2x^2 + 0x - 32 = 0 $$

$$ 2x^2 - 32 = 0 $$

$$ 2x^2 = 32 $$

$$ x^2 = 16 $$

$$ x_1 = 4, x_2 = -4 $$

Ответ: $$ x_1 = 4, x_2 = -4 $$