How do you know if the sequence #1, -2, 4, -8, ...# is arithmetic or geometric?
Because the sequence has a common ratio
Each term of an arithmetic sequence is generated by adding or subtracting a number to get the next term. The number is called the common difference
Each term of a geometric sequence is generated by multiplying or dividing by a number to get the next term. The number is called the common ratio
Let's look at the given sequence:
If you subtract the first term from the second,
If you divide the second term by the first,
Because there is a common ratio, the sequence is geometric