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
November 25th, 2013, 03:45 AM   #1
Senior Member
 
Joined: Sep 2013
From: Earth

Posts: 827
Thanks: 36

Geometric progression problem. Help

I solved this question until half.

In a geometric progression, the 5th term and the sum of the 2nd and 3rd terms are 4/9 and 16 respectively. Given that r<0, find
a( the first term and the common ratio.
jiasyuen is offline  
 
November 25th, 2013, 05:24 AM   #2
Global Moderator
 
greg1313's Avatar
 
Joined: Oct 2008
From: London, Ontario, Canada - The Forest City

Posts: 7,932
Thanks: 1127

Math Focus: Elementary mathematics and beyond
Re: Geometric progression problem. Help

ar^4 = 4/9
ar + ar^2 = 16
a = 16/(r + r^2)
16r^4/(r + r^2) = 4/9
144r^3 - 4r - 4 = 0, r = 1/3 (this is the only real root; there is an error somewhere)
a = 36.
greg1313 is offline  
November 25th, 2013, 11:22 PM   #3
Global Moderator
 
Joined: Dec 2006

Posts: 20,474
Thanks: 2039

Google finds this.
skipjack is offline  
November 26th, 2013, 12:21 AM   #4
Member
 
Joined: Aug 2013

Posts: 32
Thanks: 1

Re: Geometric progression problem. Help

I think the question should state r<1, not r<0.
evaeva is offline  
November 27th, 2013, 02:08 AM   #5
Senior Member
 
Joined: Sep 2013
From: Earth

Posts: 827
Thanks: 36

Re: Geometric progression problem. Help

I think the question is definitely incorrect.
If it state that r<1, is the answer r=1/3?
jiasyuen is offline  
November 27th, 2013, 05:47 AM   #6
Global Moderator
 
Joined: Dec 2006

Posts: 20,474
Thanks: 2039

Yes.
skipjack is offline  
Reply

  My Math Forum > High School Math Forum > Algebra

Tags
geometric, problem, progression



Thread Tools
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Geometric progression. jiasyuen Algebra 7 March 23rd, 2014 08:29 PM
Geometric progression problem jiasyuen Algebra 2 November 24th, 2013 07:31 AM
Geometric progression jiasyuen Algebra 1 November 19th, 2013 02:13 AM
Geometric Progression Ter Algebra 2 April 13th, 2012 05:34 AM
geometric progression islam Number Theory 3 July 20th, 2011 06:39 AM





Copyright © 2019 My Math Forum. All rights reserved.