Question 630801: Find the number of terms in the arithmetic sequence with the given conditions.
a1=-3, d=1/3, S=55
n=
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! Find the number of terms in the arithmetic sequence with the given conditions.
a1=-3, d=1/3, S=55
n=
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S(n) = (n/2)(a + L)
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S(n) = (n/2)(a+a+(n-1)d)
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55 = (n/2)(2a+nd - d)
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55 = (n/2)(-6 + (1/3)n - (1/3))
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55 = (n/2)(-19/3) + (1/3)n)
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55 = (n/2)[(-19+n)/3)
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55 = (n/6)(-19+n)
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330 = -19n + n^2
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n^2 - 19n - 330 = 0
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(n+11)(n-30) = 0
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n = 30
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Cheers,
Stan H.
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