## Algebra 1: Common Core (15th Edition)

The common ratio is 2 so yes, this is a geometric sequence. The explicit formula is $a_{n}$=5$\times$ $2^{n-1}$. The recursive formula is $a_{1}$=5;$a_{n}$=$a_{n-1}$ $\times$ 2
You are given the sequence 5,10,20,40.The starting value $a_{1}$=5.Find the common ratio by using the formula: R=$\frac{a2}{a1}$,R=$\frac{a4}{a3}$.Plug in the values to get the ratio: r=$\frac{10}{5}$=2 r=$\frac{40}{20}$=2 There is a common ratio, r=2. So the sequence is geometric. Substitute a1 and R into the explicit formula($a_{n}$=$a_{1}$ $\times$ $r^{n-1}$).The explicit formula is $a_{n}$=5$\times$ $2^{n-1}$. Substitute a1 and r into the recursive formula ($a_{1}$=A;$a_{n}$=$a_{n-1}$$\times$R). The recursive formula is $a_{1}$=5;$a_{n}$=$a_{n-1}$ $\times$ 2