Question

$\cos (i^{5}x)=?$

• A. $i\cos hx$
• B. $–i\cos hx$
• C. $\cos hx$
• D. $–\cos hx$

Answer 1

The correct option is C

$\cos hx$

Explanation for the correct option:

Given, $\cos (i^{5}x)$

$=\cos \left(ix\right)$ ; $\left[\mathbf{\because }\mathit{i}\mathbf{=}\sqrt{\mathbf{-}\mathbf{1}}\right]$

$=\mathit{c}\mathit{o}\mathit{s}\mathbf{}\mathit{h}\mathit{x}$ ; $\left[\mathbf{\because }\mathit{c}\mathit{o}\mathit{s}\mathbf{}\mathbf{\left(}\mathit{i}\mathit{x}\mathbf{\right)}\mathbf{}\mathbf{=}\mathbf{}\mathit{c}\mathit{o}\mathit{s}\mathbf{}\mathit{h}\mathit{x}\right]$

Hence, Option ‘C’ is Option.