int e^(-2x)/(3 + e^(-x)) dx = int e^(-x) * e^(-x)/(3 + e^(-x)) dx =
= int e^(-x)/(3 + e^(-x)) d(-e^(-x)) = -int e^(-x)/(3 + e^(-x)) d(e^(-x)) =
= | t = e^(-x) | = -int t/(3 + t) dt = -int (t + 3 - 3)/(t + 3) dt =
= -int dt + 3 * int dt/(t + 3) = -t + 3 * ln |t + 3| + C = | t = e^(-x) | =
= -e^(-x) + 3 * ln (e^(-x) + 3) + C