SOLUTION: the sum of the first four terms of an A.P.is 56.the sum of the last four terms is 112.if the first term is 11,find the number of terms.
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Question 1006645: the sum of the first four terms of an A.P.is 56.the sum of the last four terms is 112.if the first term is 11,find the number of terms.
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
the sum of the first four terms of an A.P.is 56.
If the 1st term is 11 and n = 4, solve for "d"::
(n/2)(11+(11+3d) = 56
(2)(22+3d) = 56
22+3d = 28
3d = 6
d = 2
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the sum of the last four terms is 112.
if the first term is 11,
find the number of terms.
Last term:: a(n) = 11 + (n-1)d
a(n) = 11 + (n-1)2 = 2n+9
a(n-1) = 2n+7
a(n-2) = 2n+5
a(n-3) = 2n+3
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Sum of last 4 terms = 112
8n+24 = 112
8n = 88
n = 11
Ans: # of terms is 11
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Cheers,
Stan H.
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