What are common mistakes students make with geometric sequences?
One common error is not correctly finding the value of r, the common multiplier.
For example, for the geometric sequence
A more difficult problem is this one:
Also, you might check that this is consistently true by multiplying your constant multiplier by some other term (such as the third term) to see if you get the 4th term as the answer. This will help you verify that the sequence is indeed a geometric one.