Question

$co{s}^{-1}\left[cos\left(\left(-\frac{17}{15}\right)\pi \right)\right]$ is equal to

• A. $\frac{17\pi }{15}$
• B. $\frac{13\pi }{15}$
• C. $\frac{3\pi }{15}$
• D. $-\frac{17\pi }{15}$

Answer 1

Answer: (B)

$co{s}^{-1}\left(cos\left(-\frac{17\pi }{15}\right)\right)=co{s}^{-1}\left(cos\left(\frac{17\pi }{15}\right)\right)$
$=co{s}^{-1}\left[cos\left(2\pi -\frac{13\pi }{15}\right)\right]$
$=co{s}^{-1}\left[cos\frac{13\pi }{15}\right]=\frac{13\pi }{15}$