SOLUTION: Pls help me and solve this sequence: The first and the third terms of a G.P are 5 and 80 respectively. find the 4th term

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Question 1210552: Pls help me and solve this sequence:
The first and the third terms of a G.P are 5 and 80 respectively. find the 4th term
Answer by greenestamps(13296) About Me  (Show Source):
You can put this solution on YOUR website!


The 3rd term is the 1st term, multiplied by the common ratio twice:

80=5r%5E2

Solve to find the common ratio r.

80=5r%5E2
16=r%5E2
r=4 or r=-4

The 4th term is the 3rd term, multiplied by the common ratio one more time.

Since there are two possible values for the common ratio, there are two possibilities for the 4th term.

Since the problem asks us to find "the" 4th term, perhaps the question is at an elementary level and they expect us to use only positive numbers. In that case, the 4th term is 80%2A4=320.

But another possible value for the 4th term is 80%2A%28-4%29=-320.

ANSWERS:
possibly 320
or, more correctly, either 320 or -320