int (0 +00) x * e^(-x^2/4) dx = int (0 +00) e^(-x^2/4) d(1/2 * x^2) =
= 1/2 * int (0 +00) e^(-x^2/4) d(x^2) = | t = x^2 | =
= 1/2 * int (0 +00) e^(-t/4) dt = 1/2 * (-4 * e^(-t/4))_{0}^{+00} =
= 1/2 * (-4 * lim (t->00) e^(-t/4) + 4 * e^0) = 1/2 * (-4 * lim (t->00) 1/e^(t/4) + 4) = 1/2 * (0 + 4) = 2.