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36
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avatar+99 

What is the sum of the first 21 terms of this arithmetic series?

 

-5+(-3)+(-1)+1+...

 Mar 19, 2019
edited by Nuggetguy  Mar 19, 2019
 #1
avatar+99237 
+2

The first term is  -5

The common difference is 2

So...the 21st term =   -5 + 2 (21 - 1)  =  -5 + 2(20)  = 35

 

The sum, S,  of the series  =

 

( first term + last term ) ( number of terms) / 2  =

 

(-5 + 35) (21) /2  =

 

(30) * 21 / 2 =

 

(30/2) * 21  =

 

15 * 21 =

 

315

 

 

cool cool  cool

 Mar 19, 2019

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