[ Eq(Derivative(x1, t), 0),
  Eq(Derivative(x2, t), (1*k20*k17*k15/k16*(k21/k6)**k2/(1 + (k21/k6)**k2) + (-1)*k20*k18*k15/k16*(x3/k7)**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))/((x3/k7)**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8)) + (1 + (x4/k9)**k11)/(1 + k10**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))*(x4/k9)**k11)))/k20),
  Eq(Derivative(x4, t), (1*k20*k18*k15/k16*(x3/k7)**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))/((x3/k7)**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8)) + (1 + (x4/k9)**k11)/(1 + k10**(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))*(x4/k9)**k11)) + (-1)*k20*k19*k15/k16*x4/k8/(x4/k8 + 1))/k20),
  Eq(Derivative(x5, t), 0) ]