SOLUTION: How do I find a1 for the geometric series Sn= 31.5 an= -4 r=-6?

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Question 1016837: How do I find a1 for the geometric series Sn= 31.5 an= -4 r=-6?
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
Use the formula S%5Bn%5D+=+%28a%5B1%5D+-+a%5Bn%5D%2Ar%29%2F%281-r%29.
After substitution, we get
31.5+=+%28a%5B1%5D+-+%28-4%29%2A%28-6%29%29%2F7.
Continue the steps to get the value of a%5B1%5D.