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10292011  #1 
Join Date: May 2011
Posts: 86

Urgent, Help
Alright, I have homework I need finished by tommorrow. Here is a problem or two.
7. Expressed in terms of n what is the sum of all the terms in the arithmetic sequence (n7), (n2), . . . . (n+428)? 8. If x = a/b, find (a+b)/(ab) in terms of x.
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10292011  #2 
Join Date: Mar 2005
Posts: 10,609

8.
x = a/b a = bx (a+b)/(ab) = (bx+b)/(bxb) = (b)(x+1)/(b)(x1) =(x+1)/(x1)
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10292011  #3  
Join Date: Dec 2008
Posts: 249

Quote:
The sum, Sn, of an arithmetic progression is equal to (n/2)(a1 + an) where a1 is the 1st term and an is the nth term. So we need to find n. an = a1 + d(n  1) Substituting gives: (n + 428) = (n  7) + 5(n  1) n + 428 = n  7 + 5n  5 n + 428 = 6n  12 440 = 5n n = 88 So n + 428 is the 88th term in the progression. Then the sum of the 1st 88 terms in the progression, S88, is given by: S88 = (88/2)(n  7 + n + 428) S88 = 44(2n + 421) S88 = 88n + 18524 Hope this helps. 

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