int (0 4) х * (х/8) dx = int (0 4) x^2/8 dx = 1/8 * int (0 4) x^2 dx = 1/8 * (1/3 * x^3)_{0}^{4} =
= 1/8 * (1/3 * 4^3 - 1/3 * 0^3) = 1/8 * 1/3 * 64 = 8/3.