SOLUTION: the sum of the first 6 terms of the geometric sequence 1, -2, 4, -8, ... is ?

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Question 472812: the sum of the first 6 terms of the geometric sequence 1, -2, 4, -8, ... is ?
Answer by htmentor(1343) About Me  (Show Source):
You can put this solution on YOUR website!
The common ratio is -2/1 = -2
The sum of the first n terms of a geometric sequence is:
a%281-r%5En%29%2F%281-r%29 where a is the 1st term of the sequence
Putting in n = 6, a = 1 and r = -2, we get:
Sum = %281-%28-2%29%5E6%29%281-+-2%29 -> %281-64%29%2F%281+-+-2%29
This gives Sum = -21