Question

How many terms are in the following sequence?

645700815, ..., 45, 15,5

17
18
19
16

Answer 1

18.

Explanation:

It is a geometric sequence.

The common ration is 1/3.

The nth term of a GS is a1 r(n-1)

The first term of this GS is 645700815.

Substituting to see which term is 5:

Try 16:

16th term = 645700815 * (1/3)^(16-1)

= 45

18th term = 645700815 * (1/3)^(18-1)

= 5.

So there are 18 terms.