kiara23
PLEASE HELP URGENT ! what is the 7th of the geometric sequence where a1 = 3 and a5 = -768
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amistre64
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nothings THAT urgent :/
amistre64
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we should start by remembering the general formula for a geometric sequence
amistre64
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an = a1*r^(n-1)
amistre64
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we know a1, and we know the value when n=5; from this we can grab the r value
amistre64
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can you fill those in for me?
kiara23
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kind of
amistre64
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give it a try, ill know if its right or not :)
kiara23
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the answer is going to be -3,072
amistre64
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it might be, but lets see if we can build up to that
kiara23
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ok
amistre64
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lets fill in what we are given:
a1 = 3 and a5 = -768 when n=5
an = a1 * r^(n-1)
a5 = -786 = 3 * r^(5-1)
-786/3 = r^(4)
-292 = r^(4) , 4rt the whole thing to find r
4rt(-292) = r
as is this is not a possible sequence to me. an even root can be negative across the reals
amistre64
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an even root canNOT .... i passed typing class, honest :)
amistre64
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check your problem and make sure you havent got an error in the post
kiara23
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ok
amistre64
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i got a feeling the a1 = -3 for this to work like you want it to
kiara23
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yeah
kiara23
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it is
amistre64
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4rt(292) = r then this is what we get for r
take the general formula again, knowing this r and n=7
a7 = -3 * (4rt(292))^(6) and a calculator should popout your intended results
kiara23
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can it be 12,288
amistre64
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no
amistre64
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...can what be?
amistre64
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a7 is going to be a negative value
kiara23
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-12,288
amistre64
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adjust for a mismath and i get:
a7 = -3 * 256^(6/4) = -12288 , so yes
kiara23
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can you help me with thi one ?
What is the sum of the geometric sequence -4, 28, -196 ?
amistre64
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why did you think that the answer "is going to be -3,072"?
kiara23
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i hd my ngatives wrng
kiara23
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signs
amistre64
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ah, itll prolly be best to post up a new question with that sum in it; i have to go for now