## anonymous one year ago Which of the following is the formula for the geometric sequence? 320, 80, 20, 5,... an = 320(1/4)n-1 an = 320(4)n-1--- an = 4(320)n-1 I think its the second one?

1. butterflydreamer

geometric series: |dw:1434671631437:dw|

2. butterflydreamer

so firstly, can you tell me what a1 (the first term) is?

3. anonymous

no

4. butterflydreamer

"first" term in your sequence: 320, 80, 20, 5,... what's the first number? That's your first term ...

5. anonymous

320

6. butterflydreamer

yes that's correct. Now find r (common ratio) by dividing the second term (80) by the first term (320) ... SO r = 80/ 320 = 1/4 Now we know a1 = 320, r = 1/4 So plug it into the formula and we get: $a_n= a_1r^{n-1}\rightarrow a_n =320(1/4)^{n-1}$