[ Eq(Derivative(x2, t), (-1)*k8*x2*k2/k2),
  Eq(Derivative(x3, t), ((-1)*k7*x3*k2 + 1*k8*x2*k2)/k2),
  Eq(Derivative(x4, t), ((-1)*k5*x4*k2 + 1*k7*x3*k2)/k2),
  Eq(Derivative(x5, t), (1*k5*x4*k2 + (-1)*k6*x5*k3)/k3),
  Eq(Derivative(x6, t), 1*k6*x5*k3/k4) ]