cos(z) can be expressed as

Correct option is A

​$$\text{Using \textbf{Euler's formula}, we know:} \\[6pt]e^{iz} = \cos(z) + i \sin(z) \\e^{-iz} = \cos(z) - i \sin(z) \\[10pt]\text{Adding these two equations:} \\[6pt]e^{iz} + e^{-iz} = 2 \cos(z) \\[6pt]\Rightarrow \cos(z) = \frac{1}{2} \left( e^{iz} + e^{-iz} \right)$$​​