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