[ Eq(Derivative(x1, t), (1*k5*k1*log(k2/x1)*x1 + (-1)*k5*x1*x3/(x1 + x2))/k5),
  Eq(Derivative(x2, t), (1*k5*x1*x3/(x1 + x2) + (-1)*k5*k3*x2)/k5),
  Eq(Derivative(x3, t), (1*k5*k3*x2 + (-1)*k5*k4*x3)/k5) ]