{ diff(x1, t) = (1*k9*k1*k5*x1*(x1 - 1)/k7 + (-1)*k9*k2/1*x1*(x1 - 1)*(x1 - 2)/k7^2 + 1*k9*k3*k6*k7 + (-1)*k9*k4*x1)/k9,
  diff(x2, t) = ((-1)*k9*k1*k5*x1*(x1 - 1)/k7 + 1*k9*k2/1*x1*(x1 - 1)*(x1 - 2)/k7^2 + (-1)*k9*k3*k6*k7 + 1*k9*k4*x1)/k9 }