Question 1006645
 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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