Chapter 5 - Trigonometric Functions - 5.1 Angles - 5.1 Exercises - Page 505: 98

Two positive coterminal angles: $180^\circ+360^\circ=540^\circ$ $180^\circ+2\times360^\circ=900^\circ$ Two negative coterminal angles: $180^\circ-360^\circ=-180^\circ$ $180^\circ-2\times360^\circ=-540^\circ$

In order to find the coterminal angles of a quadrantal angle, we have to add or subtract $360^\circ$. Two positive coterminal angles: $180^\circ+360^\circ=540^\circ$ $180^\circ+2\times360^\circ=900^\circ$ Two negative coterminal angles: $180^\circ-360^\circ=-180^\circ$ $180^\circ-2\times360^\circ=-540^\circ$