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# If x and y are positive, what is the value of x-y? 1) (X^2 -

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Current Student
Joined: 03 Dec 2007
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If x and y are positive, what is the value of x-y? 1) (X^2 - [#permalink]

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29 Dec 2008, 10:26
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If x and y are positive, what is the value of x-y?

1) (X^2 - Y^2) / (X+Y) = 4

2) X+Y = 7

I was trying to do this fast and thought oh 2 equations 2 variables - but you only need statement 1 to solve this and not sure why. How would you do this fast?
Current Student
Joined: 13 May 2008
Posts: 141
Schools: LBS
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Re: Yet another DS Question I can't solve [#permalink]

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29 Dec 2008, 10:40
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(X^2 - Y^2) / (X+Y) = 4

(X^2 - Y^2) = 4* (X+Y)

(x-y)*(x+y) = 4* (X+Y)

(x-y) = 4
Manager
Joined: 15 Apr 2008
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Re: Yet another DS Question I can't solve [#permalink]

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29 Dec 2008, 13:10
hardaway7 wrote:
If x and y are positive, what is the value of x-y?

1) (X^2 - Y^2) / (X+Y) = 4

2) X+Y = 7

I was trying to do this fast and thought oh 2 equations 2 variables - but you only need statement 1 to solve this and not sure why. How would you do this fast?

this is just application of a formula. x^2-y^2= (x-y)(x+y)
Current Student
Joined: 03 Dec 2007
Posts: 83
Schools: NYU STERN
Followers: 3

Kudos [?]: 6 [0], given: 0

Re: Yet another DS Question I can't solve [#permalink]

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29 Dec 2008, 13:32
thanks all - this was fairly easy- not sure why I was having trouble!!
Re: Yet another DS Question I can't solve   [#permalink] 29 Dec 2008, 13:32
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