[ Eq(Derivative(x1, t), (-1)*k7*(k1*k3*x3*x1 + k2*k4*x2*x1)/k8/k7),
  Eq(Derivative(x2, t), (1*k7*(k1*k3*x3*x1 + k2*k4*x2*x1)/k8 + (-1)*k7*k5*x2)/k7),
  Eq(Derivative(x3, t), (1*k7*k5*x2 + (-1)*k7*k6*x3)/k7),
  Eq(Derivative(x4, t), 1*k7*k6*x3/k7),
  Eq(Derivative(x5, t), 0) ]