+0  
 
+1
255
4
avatar+503 

What is the sum of the series?

 

1.

  

 

2.

.

 Mar 19, 2019
 #1
avatar+111390 
+2

I'll do the first one.....see if you can do the second

 

1.   

First term  = 2(3) - 10  =  - 4

Last term = 2(6) - 10 = 12

 

Sum   = ( first term + last term) * (number of terms)  / 2   = (-4 + 12) (6 - 3 + 1) /2  =

 

(-4 + 12) (4) /2  =

 

8 ( 4) / 2 =

 

32 / 2  =

 

16

 

 

Second one

First term =  2(1)^2 - 4  =  -2

Last term = 2(4)^2 - 4  =  32 - 4  =  28

Number of terms  =  4

 

cool  cool  cool

 Mar 19, 2019
 #3
avatar+503 
+1

Thanks, so the second would be 52 right??

whosmans  Mar 19, 2019
 #4
avatar
+1

NO!! Look at the correct answers on the bottom. CPhill made a mistake in the first one. The LAST term is 2 NOT 12.

Guest Mar 19, 2019
 #2
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0

1 - listfor(k, 3, 6, (2*k - 10)) =(-4, -2, 0, 2) = - 4

 

2 - listfor(k, 1, 4, (2*k^2 - 4)) = (-2, 4, 14, 28) = 44 

 Mar 19, 2019

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