Show that the Clausius-Clapeyron equation (4.7.7) guarantees that each of the coexistence curves at the triple point

Question:

Show that the Clausius-Clapeyron equation (4.7.7) guarantees that each of the coexistence curves at the triple point of a material "points into" the third phase; for example, the slope of the solid-vapor coexistence line has a value in-between the slopes of the the the solid-liquid and liquid-vapor coexistence lines.

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