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Medium level - sequence

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by Brent@GMATPrepNow » Thu Mar 05, 2009 1:23 pm
In the infinite sequence 13, 17, 21, 25, 29, . . . , where each term is 4 greater than the previous term, the 46th term is
(A) 183
(B) 187
(C) 191
(D) 193
(E) 197
Please note that this is not an official GMAT question; it’s my attempt to create mid-range (500 to 650-level) GMAT-style questions for this forum.
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by truplayer256 » Thu Mar 05, 2009 2:27 pm
This sequence fits the traits of an arithmetic sequence. Therefore,

an=dn+c where c=a1-d
d=4
c=13-4

an=4n+9
a46=4(46)+9=193 D
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by Brent@GMATPrepNow » Thu Mar 05, 2009 2:48 pm
Using the formula for arithmetic sequences will work.
I tend to shy away from a lot of formulae since they often are applicable in one specific situation, and some students try to apply the incorrectly to other question types.
That said, they can certainly speed things along.

Another approach is to list a few terms and look for patterns:

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