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PS2

by ddm » Mon Aug 04, 2008 2:26 pm
If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}?

1,800
1,845
1,890
1,968
2,016
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by LSB » Mon Aug 04, 2008 5:37 pm
The answer is 1968 (D)

S13 = 6*13 = 78
S28 = 6*28 = 168

The mean of a sequence with equal intervals (in this case 6) is the average of the min and max. So the mean here is 123.

There are 16 numbers in this sequence (28-13)+1 = 16

So the sum of all terms in the sequence is 16*123 = 1968
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