Question

${\cos }^{-1}\left[\cos \left(\frac{7\pi }{5}\right)\right]$ is equal to

• A. $\frac{3\pi }{5}$
• B. $\frac{2\pi }{5}$
• C. $\frac{-7\pi }{5}$
• D. $\frac{7\pi }{5}$
• E. $\frac{-2\pi }{5}$

Answer 1

Answer: (A)

${\cos }^{-1}\left(\cos \frac{7\pi }{5}\right)={\cos }^{-1}\left[\cos \left(2\pi -\frac{3\pi }{5}\right)\right]$
$={\cos }^{-1}\left(\cos \frac{3\pi }{5}\right)=\frac{3\pi }{5}$