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 August 23rd, 2008, 04:42 PM #1 Newbie   Joined: May 2008 From: Melbourne Posts: 24 Thanks: 0 sequences and series: geometric series 1. The sum of the first four terms of a geometric sequence is 312, and the sum of the next four terms is 625 times that number. Find the sum of the following four terms. 2. Find the sum of all powers of 2 between 500 and 50 000. ( i dont understand what the qq is asking ) =| thanks in advance !
 August 23rd, 2008, 06:19 PM #2 Senior Member   Joined: Jul 2008 Posts: 895 Thanks: 0 Re: sequences and series: geometric series 1. Try first please. 2. 2^8 = 256 [too little]. 2^9 = 512. 2^15 = 32 768. 2^16 = 65 536 [too much]. Sum = 2^9 + 2^10 + .... + 2^15. Use your knowledge of geometric series.
 August 24th, 2008, 01:25 AM #3 Global Moderator   Joined: Dec 2006 Posts: 17,919 Thanks: 1385 1. Let the first four terms be $a,\,ar,\,ar^2,\,ar^3.$ It's easy to proceed from there without using any formula.

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