Maths Olympiad Prep

Track / Stage 4 / 279 of 340 #539 of 1964

Problem 539

AMC 12 late, AIME early
Algebra Difficulty 4.9 Find the answer

Five. (Full marks 25 points) Solve the equation
x=(x2+3x2)2+3(x2+3x2)2 x=\left(x^{2}+3 x-2\right)^{2}+3\left(x^{2}+3 x-2\right)-2 \text {. }

A number or a short expression. Spacing and $ signs are ignored.

Official solution

Let y=x2+3x2y=x^{2}+3 x-2, then x=y2+3y2x=y^{2}+3 y-2.
Subtracting the two equations, we get
yx=(x2+3x2)(y2+3y2)=(xy)(x+y)+3(xy). \begin{aligned} y-x & =\left(x^{2}+3 x-2\right)-\left(y^{2}+3 y-2\right) \\ & =(x-y)(x+y)+3(x-y) . \end{aligned}

That is, (xy)(x+y+4)=0(x-y)(x+y+4)=0,
so xy=0x-y=0 or x+y+4=0x+y+4=0.
If xy=0x-y=0, then x=yx=y, substituting into y=x2+3x2y=x^{2}+3 x-2, we get xx
=1±3 =-1 \pm \sqrt{3} \text {. }

If x+y+4=0x+y+4=0, then y=x4y=-x-4, substituting into y=x2+3x2y=x^{2}+3 x-2, we get x=2±2x=-2 \pm \sqrt{2}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.