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How much more is the sum of the first 40 terms of the arithmetic sequence 11, 21, 31 … then the sum of the first 40 terms of the arithmetic sequence 12, 23, 34…? (1) 1600 (2) 820 (3) 780 (4) 40 |
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Answer» (2) 820 S40 = 40/2 (2 x 11 + 39 x 10) S40 = 40/2 (2 x 12 + 39 x 11) S40 - S40 = 40/2 (2 x 1 + 39 x 1) = 820 |
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