My Math Forum  

Go Back   My Math Forum > High School Math Forum > Algebra

Algebra Pre-Algebra and Basic Algebra Math Forum


Reply
 
LinkBack Thread Tools Display Modes
December 20th, 2007, 04:18 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?
ohhhh is offline  
 
December 20th, 2007, 05:42 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.
Richard André-Jeannin is offline  
December 20th, 2007, 05:42 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
milin is offline  
December 21st, 2007, 01:17 AM   #4
Global Moderator
 
Joined: Dec 2006

Posts: 19,968
Thanks: 1850

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.
skipjack is offline  
Reply

  My Math Forum > High School Math Forum > Algebra

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 09:16 AM
find a pattern/sequence westworld New Users 5 April 15th, 2012 07:44 PM
How to find the next number in a sequence westworld Real Analysis 4 January 30th, 2012 05:41 AM
To find the sequence Blickty Algebra 6 September 24th, 2011 04:15 AM
find a pattern/sequence westworld Real Analysis 0 December 31st, 1969 04:00 PM





Copyright © 2018 My Math Forum. All rights reserved.