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by simplyjat » Sat Feb 02, 2008 5:57 am
I am not sure why people prefer for upload a document, instead of just uploading the picture? Picture is great as it keeps the viewer in the same page and does not require 2-3 extra steps to just see the question...

I just ignore the questions that include another document?
Is it really that hard to upload the picture ?
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by nadontheway » Sat Feb 02, 2008 6:08 am
because some people have difficulties to upload from my computer. That's why. Come on, it will take you one more step to open the file (not too hard I guess)... I saw your post on your score. Keep fighting!!
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by Stuart@KaplanGMAT » Sat Feb 02, 2008 11:13 am
The only way we can get x^2 - y^2 as the product of those terms is if we have a difference of squares. Any other binomial product will also have an "xy" term in the middle.

The only way we can get a difference of squares from the terms shown is (x-y)(x+y).

So, we're choosing 2 of the 4 terms. 4C2 = 4!/2!2! = 4*3/2 = 6 total possible pairs.

Only 1 of the 6 possible pairs will give us what the question is asking for.

Prob = # desired outcomes / total # of possibilities = 1/6: choose (e)
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by nadontheway » Sun Feb 03, 2008 5:19 am
Oh I see Stuart!

cheers.
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how?

by meghamehta15 » Mon Feb 04, 2008 10:04 pm
how can we tell that only one will give the answer. I understand the first part about how you arrived at 6 ways but the second part is unclear..
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Re: how?

by Stuart@KaplanGMAT » Mon Feb 04, 2008 11:37 pm
meghamehta15 wrote:how can we tell that only one will give the answer. I understand the first part about how you arrived at 6 ways but the second part is unclear..
The only way we can get an answer without an xy term in the middle is if we start with a difference of squares.

For example, if we expand:

(x + y)(x + 5y)

we get:

x^2 +xy +5xy + y^2 = x^2 + 6xy + y^2

Since the question says the answer can be written in the form of:

x^2 - (by)^2,

we know that there can't be an xy term in our product. Since the only pair of terms that will give us a difference of squares is (x-y) and (x+y), only 1 out of the 6 possible pairings matches what we're looking for.
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