[ Eq(Derivative(x1, t), ((-1)*k5*(k1*x1*x2 - k2*x3) + 1*k5*k3*x3*x2 + 1*k5*k4*x3*k6)/k5),
  Eq(Derivative(x2, t), ((-1)*k5*(k1*x1*x2 - k2*x3) + (-1)*k5*k3*x3*x2)/k5),
  Eq(Derivative(x3, t), (1*k5*(k1*x1*x2 - k2*x3) + (-1)*k5*k3*x3*x2 + (-1)*k5*k4*x3*k6)/k5),
  Eq(Derivative(x4, t), 1*k5*k3*x3*x2/k5),
  Eq(Derivative(x5, t), 0),
  Eq(Derivative(x6, t), 1*k5*k4*x3*k6/k5) ]