[ Eq(Derivative(x1, t), (1*k5*k1*x2 + 1*k5*k2*x2*x1**2 + (-1)*k5*k3*x1/(x1 + 1))/k5),
  Eq(Derivative(x2, t), (1*k5*k4 + (-1)*k5*x1)/k5) ]