My Math Forum Geometric Progression
 User Name Remember Me? Password

 Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion

 August 31st, 2016, 07:09 AM #1 Newbie   Joined: Aug 2016 From: Bucharest Posts: 14 Thanks: 0 Geometric Progression In a geometric progression , (an)n>=1 , we know Sn=5^n*4 . Find b8 . I know the Sn formula but I don't know how to relate to it . Thank you !
 August 31st, 2016, 07:23 AM #2 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,683 Thanks: 2664 Math Focus: Mainly analysis and algebra What is "b8"? Do you mean $a_8$?
August 31st, 2016, 07:32 AM   #3
Newbie

Joined: Aug 2016
From: Bucharest

Posts: 14
Thanks: 0

Quote:
 Originally Posted by v8archie What is "b8"? Do you mean $a_8$?
Yes I solve arithmetic ones with a terms and geometric with b terms .

August 31st, 2016, 12:56 PM   #4
Math Team

Joined: Jul 2011
From: Texas

Posts: 3,016
Thanks: 1600

Quote:
 Originally Posted by MxMMxM Yes I solve arithmetic ones with a terms and geometric with b terms .
Quote:
 In a geometric progression , (an)n>=1 , we know Sn=5^n*4
note that the problem statement used $a_n$ to represent the nth term ...

assuming $S_n = 5^n \cdot 4$ ...

$S_7 + a_8 = S_8 \implies S_8 - S_7 = a_8 = 5^8 \cdot 4 - 5^7 \cdot 4 = 5^7 \cdot 4 (5 - 1) = 5^7 \cdot 4^2$

August 31st, 2016, 11:23 PM   #5
Newbie

Joined: Aug 2016
From: Bucharest

Posts: 14
Thanks: 0

Quote:
 Originally Posted by skeeter note that the problem statement used $a_n$ to represent the nth term ... assuming $S_n = 5^n \cdot 4$ ... $S_7 + a_8 = S_8 \implies S_8 - S_7 = a_8 = 5^8 \cdot 4 - 5^7 \cdot 4 = 5^7 \cdot 4 (5 - 1) = 5^7 \cdot 4^2$
Thank you very much !

 September 1st, 2016, 09:11 PM #6 Global Moderator   Joined: Dec 2006 Posts: 20,969 Thanks: 2222 The problem doesn't make sense. The formula $S_n = 5^n \cdot 4$ implies $a_1$ = 20, $a_2$ = 80, $a_3$ = 400, etc., which isn't a geometric progression, as 80/20 = 4, whereas 400/80 = 5.

 Tags geometric, progression

 Thread Tools Display Modes Linear Mode

 Similar Threads Thread Thread Starter Forum Replies Last Post life24 Elementary Math 2 April 2nd, 2016 06:28 AM atest0504 Algebra 12 April 2nd, 2015 01:47 PM jiasyuen Algebra 1 May 14th, 2014 08:19 AM jiasyuen Algebra 1 May 1st, 2014 03:40 AM naman Algebra 1 March 5th, 2013 06:26 AM

 Contact - Home - Forums - Cryptocurrency Forum - Top

Copyright © 2019 My Math Forum. All rights reserved.