SOLUTION: Please Help: Find the sum of the first five terms of the geometric sequence. a = 1, r = -3 A) 121 B) 61 C) -121 D) -61

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Question 201893: Please Help:
Find the sum of the first five terms of the geometric sequence.
a = 1, r = -3
A) 121
B) 61
C) -121
D) -61
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
The formula for the sum of the first "n" terms of any geometric sequence is S=%28a%28r%5En-1%29%29%2F%28r-1%29


S=%28a%28r%5En-1%29%29%2F%28r-1%29 Start with the given formula


S=%281%28%28-3%29%5E5-1%29%29%2F%28-3-1%29 Plug in a=1, r=-3, and n=5


S=%281%28-243-1%29%29%2F%28-3-1%29 Raise -3 to the 5th power to get -243


S=%281%28-244%29%29%2F%28-4%29 Subtract


S=%28-244%29%2F%28-4%29 Multiply


S=61 Divide


So the sum of the first five terms is 61 which means that the answer is B)