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Since #2 is self-explanatory, the answer has to be B or D. I tried to work out #1 and wasn't able to simplify the expression to make the answer obvious. Can someone show the simplification for #1?

Thanks!

Last edited by nervousgmat on 28 Feb 2007, 18:27, edited 1 time in total.

st1 is not sufficient:
consider x=-5 y=5 ; x=5 y=5
in both cases st1 holds but stem is either "yes" or "no"...

so this is the ultimate proof of insufficiency of st1.

it is a bit difficult to expand st1 algebraicly and people here did various mistakes....
x^2-y^2 = (x+5)(y-5) = xy + 5y - 5x -25 ..... leads to nothing.

it is of utmost importance in gmat to know when to stop seeking algebraic expansions/solutions (and start look at other directions for the answer).
in this case the (something +5)*(something -5) is put as diversion that leads you to make mistakes - either doing wrong algebraic operations or assuming wrong things....
in fact this was a "red alert" for me. the stem doesn't say anything about specific numbers, just x and y. and then comes statement 1 and adds numbers to the equation... this sounded ood to me and made me extra alerted.

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