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