SOLUTION: A1= 7, An= -20, Sn = -65 determine n and d.

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Question 754618: A1= 7, An= -20, Sn = -65 determine n and d.
Answer by tommyt3rd(5050) About Me  (Show Source):
You can put this solution on YOUR website!
%0D%0AS%5Bn%5D=%28n%2F2%29%28a%5B1%5D%2Ba%5Bn%5D%29%0D%0A
so that

%0D%0A-65=%28n%2F2%29%287%2B%28-20%29%29%0D%0A


Therefore

n=10

As for the common difference

%0D%0Ad=%28a%5Bn%5D-a%5B1%5D%29%2F%28n-1%29=%28-20-7%29%2F%2810-1%29=-3%0D%0A