Chapter 13, Trigonometric Ratios and Functions - 13.2 Define General Angles and Use Radian Measure - 13.2 Exercises - Skill Practice - Page 863: 21

$\dfrac{10\pi}{9}$ and $\dfrac{-8\pi}{9}$

a) Need to subtract $2 \pi$ from the given radian measurements. Thus, we have $\dfrac{28 \pi}{9}-2 \pi=\dfrac{28 \pi}{9}-\dfrac{18 \pi}{9}$ $=\dfrac{10\pi}{9}$ b) Need to subtract $4 \pi$ from the given radian measurements. Thus, we have $\dfrac{28 \pi}{9}-4 \pi=\dfrac{28 \pi}{9}-\dfrac{36 \pi}{9}$ $=\dfrac{-8\pi}{9}$