{ diff(x1, t) = (1*k13*k1 + (-1)*k13*(k8*x1 + k3*x1*x3/(x3 + k2)))/k13,
  diff(x2, t) = (1*k13*(k6*k3*x1*x3/(x3 + k2) + k7*x2*x3/(x3 + k2)) + (-1)*k13*(k4*x2 + k5*x3*x2))/k13,
  diff(x3, t) = (1*k13*k9*x3*log(k10/x3) + (-1)*k13*(k11*x3 - k12*x3*x2))/k13 }