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Suppose the first term is a, the second is a+r and the nth is a+(n-1)r.

Then the sum of the first five = 5a + 10r = 85

and the sum of the first six = 6a + 15r = 123

Solving these simultaneous equations, a = 3 and r = 7

So the first four terms are: 3, 10, 17 and 24

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โˆ™ 2009-10-10 20:32:45
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Q: The sum of the first 5 terms of an arithmetic series is 85. The sum of the first 6 terms is 123. Determine the first four terms of the series.?
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