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

Algebra ->  Algebra  -> Sequences-and-series -> 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       Log On

Ad: Algebrator™ solves your algebra problems and provides step-by-step explanations!
Ad: Algebra Solved!™: algebra software solves algebra homework problems with step-by-step help!

   

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(21685) 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)