(x − 3)(x + 7) − 13 = (x + 8)(x − 4) − 2;
$(x - 3)(x + 7) - 13 = x^2 + 7x - 3x - 21 - 13 = x^2 + 4x - 34$;
$(x + 8)(x - 4) - 2 = x^2 - 4x + 8x - 32 - 2 = x^2 + 4x - 34$;
$x^2 + 4x - 34 = x^2 + 4x - 34$.
Тождество верно.

16 − (a + 3)(a + 2) = 4 − (6 + a)(a − 1);
$16 - (a + 3)(a + 2) = 16 - (a^2 + 2a + 3a + 6) = 16 - (a^2 + 5a + 6) = 16 - a^2 - 5a - 6 = -a^2 - 5a + 10$;
$4 - (6 + a)(a - 1) = 4 - (6a - 6 + a^2 - a) = 4 - (5a - 6 + a^2) = 4 - 6a + 6 - a^2 + a = -a^2 - 5a + 10$;
$-a^2 - 5a + 10 = -a^2 - 5a + 10$.
Тождество верно.