Find S20 for the arithmetic series 8 + 16 + 24 + 32 + ... super confused cause my options for answers are 1) 1672 2) 1688 3) 1664 4) 1680 shouldnt the answer be 160?

Question

Stacy Rangel

Mathematics

24 July, 15:22

Find S20 for the arithmetic series 8 + 16 + 24 + 32 + ... super confused cause my options for answers are 1) 1672 2) 1688 3) 1664 4) 1680 shouldnt the answer be 160?

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Answers (1)

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Powers · Answer 1

First you have to find the 20th term ... an = a1 + (n - 1) d n = 20 a1 = 8 d = 8 now lets sub ... a20 = 8 + (20 - 1) * 8 a20 = 8 + 19 * 8 a20 = 8 + 152 a20 = 160 s20 is the sum of all the number till 20. so we will use the sum formula ... sn = n (a1 + an) / 2 s20 = 20 (8 + 160) / 2 s20 = 20 (168) / 2 s20 = 3360/2 s20 = 1680 any questions?