13. gt: $f'(x) + 2f'(-x) = \dfrac{2|x|}{x^6 + x^2 + 1} \ (1)$
Thay $x = -t$ được $f'(-t) + 2f'(t) = \dfrac{2|t|}{t^6 + t^2 + 1}$
Suy ra $f'(-x) + 2f'(x) = \dfrac{2|x|}{x^6 + x^2 + 1} \ (2)$
Lấy $(1) - (2)$ được $-f'(x) + f'(x) = 0$ hay $f'(x) = f'(-x)$
$f(-2) - f(3) = \int_{-2}^{3} f'(x) \, \mathrm{d}x$
$= \int_{2}^{-3} f'(-t) \, \mathrm{d}(-t)$
$= - \int_{2}^{-3} f'(t) \, \mathrm{d}t$
$= - (f(-3) - f(2)) = -(n - m) = m - n$