+0  
 
0
476
1
avatar+117 

What is the sum of the first five terms of the geometric series 1 − 3 + 9 − . . . ?

Hint: r ≠ 1, where a1 is the first term and r is the common ratio.

 S5 = −182

 S5 = −20

 S5 = 61

 S5 = 75

 

I know it's not B...I don't even know where to start. Help?

failurewithasmile  Apr 14, 2017
 #1
avatar+92429 
+1

Sum of a geometric series  =   a (1 - r^n) / (1 - r)

 

Where a is the first term,(1), r is the common ratio, (-3), and n is the number of terms summed

 

So  the sum of the first 5 terms  =

 

1[1 + (-3)^5] /(1 - 3)  = 61

 

 

 

cool cool cool

CPhill  Apr 15, 2017

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