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The 23^\text{rd} term in a certain geometric sequence is 16 and the 28^\text{th} term in the sequence is 24. What is the 43^\text{rd} term?

 Jan 17, 2015

Best Answer 

 #1
avatar+23254 
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A formula for the nth term of a geometric sequence is:  tn  =  a · rn - 1.

--->   16  =  a · r22 

--->   24  =  a · r27

Dividing the top equation by the bottom equation (and cancelling out the a's):  16/24  =  r22 / r27

--->  2/3  =  r-5     --->   3/2  =  r5     --->   r  =  (3/2)1/5     (which is the fifth root of 3/2)

Since  16  =  a · r22      --->   16  =  a · ( (3/2)1/5 )22    --->   a  =  16 / ( (3/2)1/5 )22  

To find the 43rd term:  a · rn - 1     --->    16 / ( (3/2)1/5 )22 · ( (3/2)1/5 )42    --->   16 · (3/2)4

 Jan 17, 2015
 #1
avatar+23254 
+5
Best Answer

A formula for the nth term of a geometric sequence is:  tn  =  a · rn - 1.

--->   16  =  a · r22 

--->   24  =  a · r27

Dividing the top equation by the bottom equation (and cancelling out the a's):  16/24  =  r22 / r27

--->  2/3  =  r-5     --->   3/2  =  r5     --->   r  =  (3/2)1/5     (which is the fifth root of 3/2)

Since  16  =  a · r22      --->   16  =  a · ( (3/2)1/5 )22    --->   a  =  16 / ( (3/2)1/5 )22  

To find the 43rd term:  a · rn - 1     --->    16 / ( (3/2)1/5 )22 · ( (3/2)1/5 )42    --->   16 · (3/2)4

geno3141 Jan 17, 2015

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