Maths Olympiad Prep

Track / Stage 4 / 317 of 340 #577 of 1964

Problem 577

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

Five. (Full marks 25 points) Solve the equation
x=(x2+3x−2)2+3(x2+3x−2)−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. Fractions can be typed as 3/2, and spacing doesn't matter.

Next problem →

Official solution

Let y=x2+3x−2y=x^{2}+3 x-2, then x=y2+3y−2x=y^{2}+3 y-2.
Subtracting the two equations, we get
y−x=(x2+3x−2)−(y2+3y−2)=(x−y)(x+y)+3(x−y). \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, (x−y)(x+y+4)=0(x-y)(x+y+4)=0,
so x−y=0x-y=0 or x+y+4=0x+y+4=0.
If x−y=0x-y=0, then x=yx=y, substituting into y=x2+3x−2y=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=−x−4y=-x-4, substituting into y=x2+3x−2y=x^{2}+3 x-2, we get x=−2±2x=-2 \pm \sqrt{2}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.