  For an evaporating black hole which is a radiation-black hole combined
system, we express the entanglement entropy and the Page time in terms of the
conformal time in the RST model. The entropy change of the black hole is nicely
written in terms of Hawking flux. Integrating the first law of thermodynamics,
we can obtain the decreasing black-hole entropy and the increasing radiation
entropy, and the entanglement entropy for this system based on the Page
argument. We also obtain analytically the critical temperature to release
black-hole information, which corresponds to the Page time, and discuss the
relation between the conserved total entropy and information recovering of the
black hole in this model.
