Chapter 13 - Sequences and Series - 13.1 Sequences - Exercises and Problems for Section 13.1 - Exercises and Problems - Page 539: 21

Geometric sequence.

In order for a sequence to be geometric, the quotient of all consecutive terms must be constant. Here: $a_1=\cos(1\pi)=-1$ $a_2=\cos(2\pi)=1$ $a_3=\cos(3\pi)=-1$ $a_4=\cos(4\pi)=1$ Thus we can see that the quotient is constant, thus it is a geometric sequence.