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Calculate the sum of the first 20 terms of an arithmetic sequence with first term 18 and common difference 11.

 Jan 15, 2020
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Let  n  =  number of terms

Let  t1  =  the first term

Let  tn  =  the nth term

Let  d  =  the common difference

Let  S  =  the sum of the terms

 

Two formulas:  S  =  n · (t1 + tn) / 2                        tn  =  t1 + (n - 1) · d

 

tn  =  t1 + (n - 1) · d     --->      tn  =  18 + (20 - 1) · 11     --->     227

 

S  =  n · (t1 + tn) / 2     --->     S  =  20 · (18 + 227) / 2     --->     2450

 Jan 15, 2020

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