Precalculus (6th Edition) Blitzer

The geometric sequence and the common ratio is $r=2$.
We are given that $a_n=2^n$ Here, we have the first four terms $a_1=2, a_2=4,a_3=8,a_4=16$ when $n=1,2,3,4$ Now, subtract each two consecutive terms as below: $a_2/a_1=2; a_3/a_2=2=1; a_4/a_3=\dfrac{16}{8}=2$ We can see that this is the geometric sequence and the common ratio is $r=2$.