int (x^3 - x) dx/(x^4 - 2 * x^2 + 7) =
= | t = x^4 - 2 * x^2 + 7;
dt = (4 * x^3 - 4 * x) dx = 4 * (x^3 - x) dx => (x^3 - x) dx = 1/4 * dt| =
= 1/4 * int dt/t = 1/4 * ln |t| + C = | t = x^4 - 2 * x^2 + 7 | =
= 1/4 * ln |x^4 - 2 * x^2 + 7| + C