About the exponential function of series

[opeth_35]

hey,

I have a question about the taylor series.I open cosθ function, I forward while making this solution but I can not write as the definition of exponential function. How Can I write this expression as an exponential function. Maybe I can not see something in it..

Do you have any idea about definition of it ?

 

[lurflurf]

cosθ=(1/2)(exp(i θ)+exp(-i θ))
where i is an imaginary unit so that i^2=-1.