{ x4 = k11 - x5,
  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 }