{ diff(x1, t) = (1*k8*(-k1)*log((x2 + x1)/k6)*x1 + (-1)*k8*(-k1)*log((x2 + x1)/k6)*k3*x1 + (-1)*k8*(-k1)*log((x2 + x1)/k6)*(k4*x1 - k5*x2))/k8,
  diff(x2, t) = (1*k8*(-k1)*log((x2 + x1)/k6)*(k4*x1 - k5*x2) + 1*k8*(-k1)*log((x2 + x1)/k6)*x2)/k8 }