Question 439933: Hello, how many terms of the sequence -19,-15,-11,... must be taken to give a sum of 98? Thanks :)
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! how many terms of the sequence -19,-15,-11,... must be taken to give a sum of 98?
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a = -19
d = 4
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S(n) = (n/2)(a + L) where L is the nth term:
L = a + (n-1)d
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Solve:
98 = (n/2)(-19 + -19 +(n-1)4)
196 = n(-38+4n-4)
196 = n(4n-42)
98 =n(2n-21)
2n^2-21n-98 = 0
(n-14)(2n+7) = 0
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Positive solution:
n = 14
Ans: 14 terms
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Cheers,
Stan H.
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