Question

If the 5th term in a geometric sequence is 162, and the common ratio is 3. What is the first term in the sequence?

Answer 1

Answer:  2

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Explanation:

To get the 6th term, we multiply the fifth term by the common ratio

6th term = (fifth term)*(common ratio)

6th term = 162*3

6th term = 486

The 7th term is found by tripling 486, and so on.

To get the fourth term, we go in reverse of this process. We'll divide 162 by 3 to get 162/3 = 54

The third term is then going to be 54/3 = 18

The second term is 18/3 = 6

The first term is 6/3 = 2

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Here's another way we can solve this question.

The nth term of a geometric sequence is a*(r)^(n-1)

We know that the common ratio is 3, so r = 3.

The 5th term is 162, meaning plugging n = 5 into that expression above leads to 162, so,

a*(r)^(n-1)

a*(3)^(n-1)

a*(3)^(5-1) = 162

a*(3)^4 = 162

a*81 = 162

81a = 162

a = 162/81

a = 2 is the first term

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The first five terms of the geometric sequence are:

2, 6, 18, 54, 162

Each time we go from left to right, we're multiplying by 3. Going in reverse (right to left), we divide by 3.

Multiplying by 1/3 is the same as dividing by 3.