Precalculus (6th Edition)

Two positive coterminal angles: $270^\circ+360^\circ=630^\circ$ $270^\circ+2\times360^\circ=990^\circ$ Two negative coterminal angles: $270^\circ-360^\circ=-90^\circ$ $270^\circ-2\times360^\circ=-450^\circ$
In order to find the coterminal angles of a quadrantal angle, we have to add or subtract $360^\circ$. Two positive coterminal angles: $270^\circ+360^\circ=630^\circ$ $270^\circ+2\times360^\circ=990^\circ$ Two negative coterminal angles: $270^\circ-360^\circ=-90^\circ$ $270^\circ-2\times360^\circ=-450^\circ$