u = y^2/(x + z)
Например du/dx = (y^2/(x + z))'_x = y^2 * (1/(x + z))'_x = y^2 * ((x + z)^(-1))'_x = y^2 * (-1) * (x + z)^(-2) = -y^2/(x + z)^2
Аналогично находим du/dy и du/dz.