[ Eq(x4, k11 - x5),
  Eq(x5, 2*k5*x1*k8/(k7 - k5*x1 + k6*k7 + k8*k5*x1 + ((k7 - k5*x1 + k6*k7 + k8*k5*x1)**2 - 4*(k7 - k5*x1)*k5*x1*k8)**(1/2))*k11) ]