# About the exponential function of series

1. Oct 4, 2011

### 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 ?

2. Oct 5, 2011

### lurflurf

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