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28 Jun 2007, 09:19
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Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
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Any suggestions for a quicker answer?
I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.
Looking for ways to cut down time on easy questions like this.
I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.
Looking for ways to cut down time on easy questions like this.
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Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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28 Jun 2007, 10:38
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using Geometric Progression formula is shorter but easy to make mistakes:
Sn = a(1-R^n)/(1-R)
n = 10
a = 1/2
F(1) = 1/2
F(2) = -1/4
F(3) = 1/8
hence R = -1/2
solve GP
Sn = 1/2*(1-(-1/2^10))/(1-(-1/2))
Sn = (1025/2048)/(3/2)
Sn = 2050/2048/3
Sn = ~1/3
the answer is (D)
Sn = a(1-R^n)/(1-R)
n = 10
a = 1/2
F(1) = 1/2
F(2) = -1/4
F(3) = 1/8
hence R = -1/2
solve GP
Sn = 1/2*(1-(-1/2^10))/(1-(-1/2))
Sn = (1025/2048)/(3/2)
Sn = 2050/2048/3
Sn = ~1/3
the answer is (D)
28 Jun 2007, 12:06
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misterJJ2u
Also D. 1 minute.
1. Find pattern. There has to be one, otherwise it wouldn't be a GMAT question. Very quickly see that this thing is just alternating positive and negative powers of 1/2:
1/2, -1/4, 1/8, -1/16....
2. Every change is smaller than the previous one. Classic GMAT game. I drew a number line:
-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
Numbers start at 1/2, then down 1/4, then up 1/8, then down 1/16. You basically bounce back and fourth, but you NEVER cross 1/4, and you never cross 1/2.
-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
...................................................... Start
............................ {---------------------- ....(Back 1/4)
............................. ------------} ...............(forward 1/8)
......................................{----- ................(back 1/16)
.......................................---} .................(forward 1/32)
....................................... {- .................. (back 1/64)
Etc, etc. It's a lot here to type out, on paper, really simple. Either way, you can see the range you're in pretty clearly.
