{ diff(x1, t) = (1*k28*k19 + (-1)*k28*(k1 + k17*2*x3*k10/x1/(k8 - x3 + k8*k9/x1 + x3*k10/x1 + ((k8 - x3 + k8*k9/x1 + x3*k10/x1)^2 - 4*x3*k10/x1*(k8 - x3))^(1/2)))*x1)/k28,
  diff(x3, t) = (1*(k4 + k18*x1 + k20*x3^k6/(k5^k6 + x3^k6)) + (-1)*k27*k2*(1 + x2)*x3)/k27 }