Maths Olympiad Prep

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Algebra Difficulty 4.5 AIME Prove it United States

Problem:
Find all real solutions (x,y)(x, y) of the system x2+y=12=y2+xx^{2}+y=12=y^{2}+x.

Solution

Solution:
We have x2+y=y2+xx^{2}+y = y^{2}+x which can be written as (xy)(x+y1)=0(x-y)(x+y-1)=0.

The case x=yx = y yields x2+x12=0x^{2}+x-12=0, hence (x,y)=(3,3)(x, y) = (3, 3) or (4,4)(-4, -4).

The case y=1xy = 1-x yields x2+1x12=x2x11=0x^{2}+1-x-12 = x^{2}-x-11=0 which has solutions x=1±1+442=1±352x = \frac{1 \pm \sqrt{1+44}}{2} = \frac{1 \pm 3\sqrt{5}}{2}. The other two solutions follow.

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