Question

Which of the following is not a geometric sequence?

1,-3,9,-27,81
-7,-5.8,-4.6,-3.4,-2.2
-100 -20 -4, 4/5, 4/25
2.8,6.72,16.128,38.7072

Answer 1

-7,-5.8,-4.6,-3.4,-2.2

Explanation:

A geometric sequence must have a common ratio.

-3 / 1 = -3

9 / -3 = -3

-27 / 9 = -3

81 / -27 = -3

The first sequence is a geometric sequence.

-5.8 / -7 = 0.829...

-4.6 / -5.8 = 0.793...

The second sequence is not a geometric sequence (it is an arithmetic sequence with a common difference of 1.2).