Calculus: Early Transcendentals 8th Edition

$$a_n = \sin{(\frac{n\pi}{2})}$$
Our first term is $a_1 = 1$. The sequence resembles the relationship of $\sin(x)$ at evaluations of $x = n\frac{\pi}{2}$ as $n = 1, 2, 3, 4, \cdots.$ Hence $$a_n = \sin{(\frac{n\pi}{2})}$$