SOLUTION: In a geometric progression, the common ratio is -2 and the 8th term is -640. What is the first term?

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Question 922690: In a geometric progression, the common ratio is -2 and the 8th term is -640. What is the first term?
Answer by srinivas.g(540) About Me  (Show Source):
You can put this solution on YOUR website!
Formula :n th term = a%2Ar%5E%28n-1%29
where a = first term
r= common ratio =-2
we need to figure out 8 th term. so put n=8 in the above equation
8 th term = a%2Ar%5E%288-1%29
++-640++=+a%2A%28-2%29%5E%288-1%29
++-640++=+a%2A%28-2%29%5E%287%29
++-640++=+a%2A%28-128%29
divide with -128 on both sides
+%28+-640%2F-128%29 =%28+a%2A%28-128%29%29%2F%28-128%29
5= a
result : first term (a) =5