Question

If the first 4 terms of a geometric sequence are {7, 21, 63, 189), then the formula for the nth term in the sequence is?

Answer 1

Answer: 7 × ( 3^n - 1 )

Step-by-step explanation:

This is a geometric progression which has its nth term to be

Tn = ar^n- 1. Now from the question given, the sequence are 7, 21, 63, 189. This is an example of a finite sequence, To find the common ratio, you divide the second term by the first term. So

r = 21/7

= 3. , a = 7

Now to find the nth term put the values in the formula above.

Tn =ar^n-1

= 7(3)^n - 1)

= 7 × ( 3^n - 1 ).

Note, it is not 21^n - 1.