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In an arithmetic series, the 6th term is 39.  In the same arithmetic series, the 19th term is 7.8.  Work out the sum of the first 25 terms of the arithmetic series.

 Sep 24, 2021
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19th term  -  6th term ==13 terms

7.8  -  39 ==- 31.2 sum of difference of the 13 terms

 

- 31.2 / 13 == - 2.4 - this is the common difference.

39  -  ( - 2.4 * 5) ==39  +  12 ==51 - this is the first term.

 

Your sequence looks like this:

 

51 , 48.6 , 46.2 , 43.8 , 41.4 , 39 , 36.6 , 34.2 , 31.8 , 29.4 , 27 , 24.6 , 22.2 , 19.8 , 17.4 , 15 , 12.6 , 10.2 , 7.8 , 5.4 , 3 , 0.6 , -1.8 , -4.2 , -6.6 , >>Number of terms = 25>>Total Sum == 555

 Sep 24, 2021

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