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