Question

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

1. a1 = 7;an = 3 • an - 1
2. a1 = 7;an = 1/3 • an - 1
3. a1 = 3;an = 7 • an - 1
4. a1 = 21;an = 7 • an - 1

Answer 1

The top choice is the answer. a1=7; an= 3*a_(n-1)

Explanation:

Begin by finding out what the terms are multiplied by. It is a geometric sequence so multiplication is involved.

Take the 3rd term (63) and divide it by the second term.

63/21 = 3

What this means is that each term in the sequence is multiplied by 3 to get to the next term.

The first term is 7

So the answer is an = 3* a_n-1

The answer must be the top one. See if it works.

a2 = 3*a_(2 -1 )

a2 = 3 * a1

a2 = 3 * 7

a2 = 21 which it does.