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#1
11-20-2007, 10:09 PM
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Sum of a geometric series
Find the sum (s) of a geometric series for which A1=48, An=3, and r=-1/2
Now, the formula is s = A1/1-r, so 48/1.5 = 32 is what I get. However, the choices are – 99, 33, 99, or 15 Why does the formula not use An and should I chose 33 since it is the closest? I am lost. 
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#2
11-20-2007, 10:15 PM
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Re: Sum of a geometric series
[ QUOTE ]
Find the sum (s) of a geometric series for which A1=48, An=3, and r=-1/2 Now, the formula is s = A1/1-r, so 48/1.5 = 32 is what I get. However, the choices are – 99, 33, 99, or 15 Why does the formula not use An and should I chose 33 since it is the closest? I am lost. [/ QUOTE ] 48 + (-24) + 12 + (-6) + 3 = 33.
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#3
11-20-2007, 10:18 PM
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Re: Sum of a geometric series
[ QUOTE ]
Find the sum (s) of a geometric series for which A1=48, An=3, and r=-1/2 Now, the formula is s = A1/1-r, so 48/1.5 = 32 is what I get. [/ QUOTE ] The formula you site is true if you go to infinity, that is you keep summing terms till infinity 48 - 24 + 12 - 6 + 3 - 3/2 + 3/4 - 3/8 + 3/16 - ... But the problem tells you An = 3, hence you stop the sum at 3. The formula is different in that case. Either way, to sum five terms, there's no need of a formula.
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