[ Diff(x[1], t) = (1*k[5]*k[1]*x[2] + 1*k[5]*k[2]*x[2]*x[1]^2 + (-1)*k[5]*k[3]*x[1]/(x[1] + 1))/k[5],
  Diff(x[2], t) = (1*k[5]*k[4] + (-1)*k[5]*x[1])/k[5] ]