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What is the 21st term in the following geometric sequence? –8, 16, –32, 64, –128,

What is the 21st term in the following geometric sequence?

–8, 16, –32, 64, –128, . .

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Francisco E. | Francisco; Civil Engineering, Math., Science, Spanish, Computers.Francisco; Civil Engineering, Math., Sci...
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-8, 16, –32, 64, –128 
 
This corresponds to the following form of sequence:
a X (-2n-1), being a = 8 and r ( common ratio) = -2
then the 21 term will be
a X r(21-1)= 8 x (-2)20 = -8388608
 
Philip P. | Effective and Affordable Math TutorEffective and Affordable Math Tutor
4.9 4.9 (385 lesson ratings) (385)
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The terms in the sequence are:
 
-2n-1*8,  n=1,2,3,...      (Since 8=23,we could also write the sequence as -2n+2, n=1,2,3,...)
 
The 21st term occurs for n=21:
 
-221-1*8 = -220*8
 
Use your calculator to compute the answer.