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 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
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 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
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 Originally Posted by MxMMxM Yes I solve arithmetic ones with a terms and geometric with b terms .
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 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
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 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 Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded 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

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