[ Diff(x[1], t) = ((-1)*k[1]*x[1] + 1*k[5]*x[1])/k[13],
  Diff(x[2], t) = (1*(k[2] + k[3]*x[1]/(x[1] + k[4]))*x[1] + (-1)*k[6]*x[2] + 1*k[10]*x[2])/k[13],
  Diff(x[3], t) = (1*(k[7] + k[8]*x[2]/(x[2] + k[9]))*x[2] + (-1)*k[11]*x[3])/k[13] ]