Hey guys. Can u pls help me with this:

The first and fourth terms of a geometric series are 18 and - 2/3 respectively.

Find

a) the sum of the first n terms of the series

b) the sum to infinity of the series

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Hey guys. Can u pls help me with this:

The first and fourth terms of a geometric series are 18 and - 2/3 respectively.

Find

a) the sum of the first n terms of the series

b) the sum to infinity of the series

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## Answers (1)

Let the series be

a + ar + ar^2 + ar^3 + . . .

From the given information

a = 18, ar^3 = -2/3 ----> r^3 = (-2/3) / 18 = -1/27

When you have the value of r

a) Use the formula S(n) = a(1 - r^n)/(1 - r)

b) Use the formula S(inf) = a/(1 - r)

Can you finish from there?