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Note on explicit form of entanglement entropy in the RST model

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.