Chapter 13, Trigonometric Ratios and Functions - 13.2 Define General Angles and Use Radian Measure - 13.2 Exercises - Skill Practice - Page 863: 16

$615^{\circ}$ and $-105^{\circ}$

In order to get the co-terminal angle we will have to add and subtract $360^{\circ}$ in the given angle. We need to add $360^{\circ}$ in the given radian measurements. Thus, we have $255^{\circ}+360^{\circ}=615^{\circ}$ Now, subtract $360^{\circ}$ from the given radian measurements. Thus, we have $255^{\circ}-360^{\circ}=-105^{\circ}$