SOLUTION: AnAP has 15 term and a common difference of -3 find the first and last term if its sum is 120

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Question 998907: AnAP has 15 term and a common difference of -3 find the first and last term if its sum is 120
Answer by mananth(16946) About Me  (Show Source):
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S%5Bn%5D+=+%28n%2F2%29%282a%2B%28n-1%29d%29
n=15
a=?
d=-3
S%5B15%5D=120
S%5B15%5D=+%2815%2F2%29%282a%2B%2815-1%29%28-3%29%29
=(15/2)(2a-42)
120*2 = 15(2a-42)
120*2/15 = 2a-42
8*2=2a-42
16+42=2a
58=2a
a=29 First term

t%5Bn%5D=a%2B%28n-1%29d
t%5B15%5D=+29%2B%2814%29%28-3%29
=29-41
=-12
t%5B15%5D+=+-12