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If x + y = 2, and x^2 - xy - 10 - 2y^2 = 0, what does x - 2y

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If x + y = 2, and x^2 - xy - 10 - 2y^2 = 0, what does x - 2y [#permalink] New post 27 Oct 2006, 16:15
If x + y = 2, and x^2 - xy - 10 - 2y^2 = 0, what does x - 2y equal?

A) 0
B) 1
C) 2
D) 5
E) 10

Can I substitute -y + 2 for x in the second equation? If not, why?
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 [#permalink] New post 27 Oct 2006, 19:47
D.

the long equation is basically (x+y)(x-2y) = 10.
then u get the answer instantly.
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 [#permalink] New post 28 Oct 2006, 02:20
yep 5

(x + y)(x - 2y) = 10

x^2 + -2xy + xy - 2y^2 = 10

x^2 - xy - 2y^2 = 10
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 [#permalink] New post 28 Oct 2006, 09:45
Thanks for the answers. But I wanted to know why substitution didn't seem to work here. I tried substituting -y + 2 for x in the second equation, x^2 - xy - 10 - 2y^2 = 0.

I do realize that the 2nd equation is a quadratic and we can and prob should solve this prob. using the above method. I'm just curious whether any alternate solutions would work.
  [#permalink] 28 Oct 2006, 09:45
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If x + y = 2, and x^2 - xy - 10 - 2y^2 = 0, what does x - 2y

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