Find the sum of the first 20 terms of the sequence -7-3, ...
Find the sum of the first 20 terms of the sequence -7-3, 1, ......
620
660
690
1240
Correct answer is A
d = -3 - (-7) = 4
S20 = 202{2(-7) + (20 - 1) 4}
= 10(- 14 + 76)
= 620
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