Question

What is the recursive rule for this geometric sequence? 7, 21, 63, 189, …

A. a1=7;an=3⋅an−1


B. a1=7;an=13⋅an−1


C. a1=21;an=7⋅an−1


D. a1=3;an=7⋅an−1

PPPLLLEASE HELP

Answer 1

A

Explanation:

a1=7:an=3*an-1

Answer 2

A. a1=7;an=3⋅an−1

Explanation:

Clearly, the first term is 7, so a1=7.

The ratio of any term to the one before it is 21/7 = 63/21 = 189/63 = 3, so that relation is ...

`a_n=3\cdot a_(n-1)`

These values of a1 and an match the first choice.