Use the fundamental identities to find the value of the trigonometric function.
-$$\cos \frac { 17 \pi } { 72 } = \cos \frac { \pi } { 9 } \cos \frac { \pi } { 8 } - \sin \frac { \pi } { 9 } \sin \frac { \pi } { 8 }$$

Question

Quizplus · Accepted Answer

This equation is an application of the cosine addition formula, $\cos(A + B) = \cos A \cos B - \sin A \sin B$, with $A = \frac{\pi}{9}$ and $B = \frac{\pi}{8}$, making $A + B = \frac{\pi}{9} + \frac{\pi}{8} = \frac{8\pi + 9\pi}{72} = \frac{17\pi}{72}$.

Use the Fundamental Identities to Find the Value of the Trigonometric