Chapter 11 - Infinite Sequences and Series - 11.1 Sequences - 11.1 Exercises - Page 744: 6

$0,-1,0,1,0$

To find the first five terms of the sequence $a_n = cos(\frac{n\pi}{2})$, we must plug in $n=1, n=2, n=3, n=4,$ and $n=5.$ $a_1 = cos(\frac{\pi}{2})=0$ $a_2 =cos(\frac{2\pi}{2})=-1$ $a_3 = cos(\frac{3\pi}{2})=0$ $a_4 = cos(\frac{4\pi}{2})=1$ $a_5 = cos(\frac{5\pi}{2})=0$ Hence we see that the first five terms are $0,-1,0,1,0$.