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Help 17,18

NotSoSmart  May 17, 2018
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 #1
avatar+86613 
+1

17

 

5th term  = 140

7th term = 35

 

We have that  

 

(140)r^2  = (35)       where r  is the common ratio

 

Divide both sides by  140

 

r^2  = 35/140  =  1/4        take both roots

 

r = +/-  sqrt  (1/4)   =  +/- 1/2

 

So....note that the 6th term could be  (140)(1/2)  = 70

Or.....the 6th term could be (140) (-1/2)  =  -70

Thus.....  ± 70   is correct

 

 

 

cool cool cool

 
CPhill  May 17, 2018
 #2
avatar+86613 
+1

18

 

Note that the  first term  is 2(1) + 1 =  3   and the last term  is 2(12) + 1  = 25

 

So...the sum is

 

[ first term + last term ]  *  [ number of terms ] / 2  =

 

[ 3 + 25 ] *  [ 12]  / 2  =

 

28 *  6  =

 

168

 

 

 

 

cool cool cool

 
CPhill  May 17, 2018
 #3
avatar
0

The Answer is 168.

 
Guest May 17, 2018
edited by Guest  May 17, 2018

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