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Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

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Re: If x^2 + y^2 = 29, what is the value of (x - y)^2 ? [#permalink]
04 Feb 2014, 00:50

1

This post received KUDOS

Expert's post

SOLUTION

If x^2 + y^2 = 29, what is the value of (x - y)^2 ?

Given: x^2+y^2=29. Question: (x-y)^2=x^2-2xy+y^2=(x^2+y^2)-2xy=29-2xy=? So, basically we should find the value of xy

(1) xy = 10. Directly gives us the value we needed. Sufficient.

(2) x = 5. Now even if we substitute the value of x in x^2+y^2=29 we'll get two values for y: 2 and -2, hence two values for (x-y)^2: 9 and 49. Not sufficient.

Re: If x^2 + y^2 = 29, what is the value of (x - y)^2 ? [#permalink]
04 Feb 2014, 04:13

1

This post received KUDOS

x^2 + y^2 = 29. (x-y)^2 = ? or x^2 + y^2 - 2xy = ? From St1) xy = 10, so x^2 + y^2 - 2xy = 29 - 2*10 = 9 --> Suff From St2) x = 5. then y = 2 or -2. Results in two values --> not suff

Re: If x^2 + y^2 = 29, what is the value of (x - y)^2 ? [#permalink]
08 Feb 2014, 04:33

Expert's post

SOLUTION

If x^2 + y^2 = 29, what is the value of (x - y)^2 ?

Given: x^2+y^2=29. Question: (x-y)^2=x^2-2xy+y^2=(x^2+y^2)-2xy=29-2xy=? So, basically we should find the value of xy

(1) xy = 10. Directly gives us the value we needed. Sufficient.

(2) x = 5. Now even if we substitute the value of x in x^2+y^2=29 we'll get two values for y: 2 and -2, hence two values for (x-y)^2: 9 and 49. Not sufficient.