SOLUTION: Find the sum of arithmitic sequence. The 3rd term is -12 and the 7th term is 8

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Question 976138: Find the sum of arithmitic sequence. The 3rd term is -12 and the 7th term is 8
Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
system%28a%2B%283-1%29d=-12%2Ca%2B%287-1%29d=8%29
system%28a%2B2d=-12%2Ca%2B6d=8%29
system%28a=-2d-12%2Ca=-6d%2B8%29
-
-2d-12=-6d%2B8
d%2B6=3d-4
6%2B4=2d
10=2d
highlight_green%28d=5%29
-
a=-2d-12
a=-2%2A5-12
a=-10-12
highlight_green%28a=-22%29

General term for the sequence is highlight%28-22%2B%28n-1%29%2A5%29.

You did not say what limits for the sum you want. You can lookup the formula for sum of an arithmetic sequence or you can derive it yourself. A crude way to show the formula is like,

%28n%2F2%29%28FirstTerm%2BLastTerm%29