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Algebra Difficulty 5.4 AIME, harder Prove it Croatia

Find all pairs of real numbers (x,y)(x, y) satisfying the following system:
x+y2=y3, x + y^2 = y^3,
y+x2=x3. y + x^2 = x^3.

Solution

Subtracting the given equations, we obtain yx+x2y2=x3y3y - x + x^2 - y^2 = x^3 - y^3, which is equivalent to
(xy)(1+x+y)=(xy)(x2+xy+y2), (x - y)(-1 + x + y) = (x - y)(x^2 + xy + y^2),
implying that either xy=0x - y = 0 or x+y1=x2+xy+y2x + y - 1 = x^2 + xy + y^2.

In the latter case, x+y1=x2+xy+y2x + y - 1 = x^2 + xy + y^2 is equivalent to
y2+(x1)y+x2x+1=0.() y^2 + (x-1)y + x^2 - x + 1 = 0. \quad (*)
This is a quadratic equation in yy. Its discriminant is
D=(x1)24(x2x+1)=3x2+2x3, D = (x - 1)^2 - 4(x^2 - x + 1) = -3x^2 + 2x - 3,
which is a quadratic polynomial in xx – its discriminant in turn is 32<0-32 < 0, so the starting discriminant is D<0D < 0 for every real number xx, which implies that the equation ()(*) does not have any real solution yy. Hence there are no solutions in this case.

In the remaining case, y=xy = x so both equations of a given system are reduced to x+x2=x3x + x^2 = x^3, i.e. x(x2x1)=0x(x^2 - x - 1) = 0. This equation has three solutions x1=0x_1 = 0, x2,3=1±52x_{2,3} = \frac{1 \pm \sqrt{5}}{2}.

The only solutions are (x,y){(0,0),(1+52,1+52),(152,152)}(x, y) \in \{(0, 0), (\frac{1+\sqrt{5}}{2}, \frac{1+\sqrt{5}}{2}), (\frac{1-\sqrt{5}}{2}, \frac{1-\sqrt{5}}{2})\}.

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