What is the value of xy? : DS Archive
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# What is the value of xy?

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What is the value of xy? [#permalink]

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22 Mar 2007, 12:28
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Can anyone help me make sense of this one? Thanks in advance for your help..

What is the value of xy?

(1) (x + y)^2 = 37
(2) (x - y)^2 = 17

I came up with 'E', but after checking the answer, it was "C". Can someone please explain why?
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22 Mar 2007, 12:47
Here's my solution:

1) (x + y)^2 = 37
x^2 + y^2 +2xy = 37

One equation and 2 variables - insuff.

2) (x - y)^2 = 17
x^2 + y^2 - 2xy = 17
One equation and 2 variables - insuff.

working with 1) & 2)
1) - 2) = 20 (37 - 17)
x^2 + y^2 +2xy - ( x^2 + y^2 - 2xy ) = 20
solve and you get xy = 5...............suff. so C
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23 Mar 2007, 06:59
Vikramjit_01

Thank you very much. After going over it once more and seeing your explanation, it's clearer now, much appreciated!
23 Mar 2007, 06:59
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