SOLUTION: Find the sum of the first 20 terms in the arithmetic series 15 + 4 - 7

SOLUTION: Find the sum of the first 20 terms in the arithmetic series 15 + 4 - 7 - ...

Question 1035866: Find the sum of the first 20 terms in the arithmetic series 15 + 4 - 7 - ...

Answer by ikleyn(52781) (Show Source): You can put this solution on YOUR website!
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Find the sum of the first 20 terms in the arithmetic series 15 + 4 - 7 - ...
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Use the formula for the sum of the first n terms of an arithmetic progression

= =

where is the first term, d is the common difference and n is the number of terms (see the lesson Arithmetic progressions in this site).

In your case = 15, d = -11, n = 20.

Plug in this data in the formula and get

= .

Make calculations.

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