Question

The value of $\cos \left(2{\cos }^{-1}0.8\right)$ is

• A. $0.48$
• B. $0.96$
• C. $0.6$
• D. None of these

Answer 1

The correct option is D None of these

Since, $2{\cos }^{-1}x={\cos }^{-1}(2x^2-1)$
Therefore, $2{\cos }^{-1}0.8={\cos }^{-1}(2\times 0.64-1)={\cos }^{-1}\left(0.28\right)$
$\Rightarrow \cos \left({\cos }^{-1}0.8\right)=\cos \left({\cos }^{-1}0.28\right)=0.28$