<PRE><B>Question 691886</B>
&quot;How many terms are in the sum of an arithmetic sequence if the first term is 21, the common difference is 9 and the sum is 1,260?&quot; 
I know that (a one) is 21, d=9 and n=1260 
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Sum Formula:
S(n) = (n/2)(a(1)+(a(n)))
1260 = (n/2)(21 + [21+ (n-1)9]
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1260 = (n/2)[42+9n-9]
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2520 = (n)[33+9n]
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2520 = 33n + 9n^2
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3n^2 + 11n - 840 = 0
(n-15)(3n+ 56) = 0
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Positive solution:
n = 15
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cheers,
Stan H.
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