SOLUTION: An A.p has 15 terms and a common difference of -3 find its first and last terms if its sum -120

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Question 1053000: An A.p has 15 terms and a common difference of -3 find its first and last terms if its sum -120
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!
S = (n/2)(2a + (n-1)d)
-120 = 15/2 (2a + 14(-3))
-16 = 2a - 42
26 = 2a
a1 = 13
a%5Bn%5D=a%5B1%5D%2B%28n-1%29%2Ad
a15 = 13 + (-42)
a15 = -29
Checking
S%5Bn%5D=%28n%28a%5B1%5D%2Ba%5Bn%5D%29%29%2F2
-120 = 15(13-29)/2 = 15(-16)/2 = -120 CHECKS