
Algebra PreAlgebra and Basic Algebra Math Forum 
 LinkBack  Thread Tools  Display Modes 
December 20th, 2007, 03:18 AM  #1 
Newbie Joined: Nov 2007 Posts: 6 Thanks: 0  find the sum of gemetric sequence
∑ (5i+10) , i=1 bottm, 30 top (sorry, don't know how to make it look nicer) how do i find the sum of this? what is the equation and how do i find r? 
December 20th, 2007, 04:42 AM  #2 
Senior Member Joined: Oct 2007 From: France Posts: 121 Thanks: 1 
The sequence is arithmetic, not geometric. S=(10+5*1)+(10+5*2)+...+(10+5*30) ( there are 30 terms) =(10+10+..+10)+5*(1+2+..+30) =30*10+5*(1+2+..+30) You certainly knows that: 1+2+..+n=n(n+1)/2. From this, 1+2+..+30=30*31/2=465 and S=300+5*465=2 625. 
December 20th, 2007, 04:42 AM  #3 
Senior Member Joined: May 2007 Posts: 402 Thanks: 0 
And r is? You could use the fact that the sum operator is linear and that: a ∑i= a(a+1)/2 i=1 a ∑c= a c i=1 
December 21st, 2007, 12:17 AM  #4 
Global Moderator Joined: Dec 2006 Posts: 19,526 Thanks: 1750 
The sum of an arithmetic progression equals the product of the number of terms and the arithmetic mean of the first and last terms. For the problem given, that's 30 × (15 + 160)/2, i.e., 2625. 

Tags 
find, gemetric, sequence, sum 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Sequence: find a2  Albert.Teng  Algebra  2  October 1st, 2012 08:16 AM 
find a pattern/sequence  westworld  New Users  5  April 15th, 2012 06:44 PM 
How to find the next number in a sequence  westworld  Real Analysis  4  January 30th, 2012 04:41 AM 
To find the sequence  Blickty  Algebra  6  September 24th, 2011 03:15 AM 
find a pattern/sequence  westworld  Real Analysis  0  December 31st, 1969 04:00 PM 