arithematic sequence

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LFalken: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Wed May 13, 2009 5:40 am; Location: MN; Thanked: 1 times; GMAT Score:580

by LFalken » Tue Jun 16, 2009 9:36 am

In the arithmetic sequence t1, t2, t3........tn....t1=23 and tn = tn-1 -3 for each n>1. What is the value of n when tn=-4?

answer is = 10

Can someone explain this one to me? Thank you!

LFAL

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Source: Beat The GMAT — Problem Solving | Tue Jun 16, 2009 9:36 am

p2pg: Senior | Next Rank: 100 Posts; Posts: 35; Joined: Mon May 11, 2009 12:11 pm; Thanked: 3 times

by p2pg » Tue Jun 16, 2009 10:27 am

If t1=23, then t2=t1-3=20...so the sequence will be
23,20,17,14,11,8,5,2,-1,-4

If tn=-4, then n=10

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tohellandback: Legendary Member; Posts: 752; Joined: Sun May 17, 2009 11:04 pm; Location: Tokyo; Thanked: 81 times; GMAT Score:680

by tohellandback » Tue Jun 16, 2009 8:37 pm

in an AP tn=a+(n-1)d
-4=23+(n-1)*(-3)
(n-1)=-27/-3
n=9+1=10

The powers of two are bloody impolite!!

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