Difference between revisions of "2007 Cyprus MO/Lyceum/Problem 1"

Revision as of 20:26, 6 May 2007

If $x-y=1$ , then the value of the expression $K=x^2+x-2xy+y^2-y$ is

$\mathrm{(A) \ } 2\qquad \mathrm{(B) \ } -2\qquad \mathrm{(C) \ } 1\qquad \mathrm{(D) \ } -1\qquad \mathrm{(E) \ } 0$

$x^2+x-2xy+y^2-y=x^2-2xy+y^2+x-y=(x-y)^2+x-y=(1)^2+1=2\Longrightarrow\mathrm{ A}$

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	==See also==		==See also==
−	{{CYMO box\|year=2007\|l=Lyceum\|before=First ~~question~~\|num-a=2}}	+	{{CYMO box\|year=2007\|l=Lyceum\|before=First Question\|num-a=2}}