{ diff(x1, t) = (1*k9*k1*(1 - x1)*x1 + (-1)*k9*k2*k3*x3*x1)/k9,
  diff(x2, t) = (1*k9*k4 + 1*k9*k5*x1*x2/(k6^2 + x1^2) + (-1)*k9*k7*x2)/k9,
  diff(x3, t) = (1*k9*k8 + 1*k9*x3*x2 + (-1)*k9*x3 + (-1)*k9*(1 - k2)*k3*x3*x1)/k9 }