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The first term of an arithmetic sequence is -8. Find the 20th term if the sum of the first 20 terms is 790.

 Jan 6, 2020
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Sum, S, of the  first 20  terms  is given by

 

S   =   ( a1  +  a20) * (n /2)   

 

790  =  (-8 +  a20)  * (20/2)

 

790  = (-8 + a20) * 10               divide both sides by 10

 

79 =  -8 +  a20       add 8 to both sides

 

87   = a20

 

 

cool cool cool

 Jan 6, 2020

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