Asked by Melody
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
calculate sum of the first 50 term
Answers
Answered by
mathhelper
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
---> (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
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