+0  
 
0
114
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
Sort: 

1+0 Answers

 #1
avatar+75362 
+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

11 Online Users

avatar
avatar
avatar
We use cookies to personalise content and ads, to provide social media features and to analyse our traffic. We also share information about your use of our site with our social media, advertising and analytics partners.  See details