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Worked solution

Sum twenty arithmetic terms

Find the sum of the first 2020 terms of the AP 7,13,19,25,…7,13,19,25,\ldots.

CJC · Sequences and Series · Example 19

Answer

12801280.

Compare your reasoning

Identify the first term, difference and count

a=7,d=6,n=20,S20=202[2(7)+(20−1)(6)]=1280a=7,\quad d=6,\quad n=20,\qquad S_{20}=\frac{20}{2}[2(7)+(20-1)(6)]=1280

The source uses a=7,d=6,n=20. This asks for a sum, rather than the twentieth term.

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