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