the sum of the first five terms of an AP is -15
---> (5/2)(2a + 4d) = -15
2a + 4d = -6
a +2d = -3 , #1
the sum of the next six terms is 279.
---> sum of first 11 terms = 279 + (-15)
(11/2)(2a + 10d) = 264
2a + 10d = 48
a + 5d = 24 , #2
subtracting #1 from #2
3d = 27
d = 9
then a = -21
sum of 50 = ...... use your formula
the sum of the first five terms of an AP is -15 and the sum of the next six terms is 279.
calculate sum of the first 50 term
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