Chapter 10.3, Problem 5E

To determine

To show: the lemma 10.10 $\cos \frac{2\pi jk}{n}$ as $\frac{1}{2}\left[e^{i\frac{2\pi jk}{n}}+e^{-i\frac{2\pi jk}{n}}\right]$ and write in terms of $\omega =e^{-i2\pi /n}$ equation for lemma 10.1 can applied in the condition.