Question

Find cos^–1$\left\{ \frac{1}{\sqrt{2}}\left( \cos\frac{9\pi}{10}–\sin\frac{9\pi}{10} \right) \right\}$ =

• A. $\frac{3\pi}{20}$
• B. $\frac{7\pi}{20}$
• C. $\frac{7\pi}{10}$
• D. $\frac{17\pi}{20}$

Answer 1

Answer: (D)

$\frac{17\pi}{20}$

Answer 2

cos^–1$\left\lbrack \cos\left( \frac{\pi}{4} + \frac{9\pi}{10} \right) \right\rbrack$  cos^–1$\left\lbrack \cos\frac{46\pi}{40} \right\rbrack$

2p – $\frac{46\pi}{40}$ = $\frac{34\pi}{40}$ = $\frac{17\pi}{20}$