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Algebra Difficulty 5.6 AIME, harder Prove it Croatia

Determine all pairs (x,y)(x, y) of real numbers such that x+y=x2+y2=x3+y3x + y = x^2 + y^2 = x^3 + y^3.

Solution

Let S=x+yS = x + y and P=xyP = x y.

We are given:

S=x+y=x2+y2=x3+y3 S = x + y = x^2 + y^2 = x^3 + y^3

Recall:
x2+y2=(x+y)22xy=S22P x^2 + y^2 = (x + y)^2 - 2 x y = S^2 - 2P
x^3 + y^3 = (x + y)^3 - 3 x y (x + y) = S^3 - 3 P S

So, the system becomes:

1. S=S22PS = S^2 - 2P
2. S=S33PSS = S^3 - 3 P S

From (1):
S=S22P    S2S2P=0    2P=S2S    P=S2S2 S = S^2 - 2P \implies S^2 - S - 2P = 0 \implies 2P = S^2 - S \implies P = \frac{S^2 - S}{2}

From (2):
S=S33PS    S33PSS=0    S3S3PS=0 S = S^3 - 3 P S \implies S^3 - 3 P S - S = 0 \implies S^3 - S - 3 P S = 0
Substitute PP from above:
S3S3SS2S2=0 S^3 - S - 3 S \cdot \frac{S^2 - S}{2} = 0
Multiply both sides by 2:
2S32S3S(S2S)=0 2 S^3 - 2 S - 3 S (S^2 - S) = 0
2S32S3S3+3S2=0 2 S^3 - 2 S - 3 S^3 + 3 S^2 = 0
(2S33S3)+(3S2)2S=0 (2 S^3 - 3 S^3) + (3 S^2) - 2 S = 0
(S3)+3S22S=0 (-S^3) + 3 S^2 - 2 S = 0
S33S2+2S=0 S^3 - 3 S^2 + 2 S = 0
S(S23S+2)=0 S (S^2 - 3 S + 2) = 0
S(S1)(S2)=0 S (S - 1)(S - 2) = 0

So the possible values for SS are 00, 11, or 22.

Now, for each SS, compute PP:

- If S=0S = 0:
P=0202=0 P = \frac{0^2 - 0}{2} = 0
So xx and yy are roots of t20t+0=0    t2=0    t=0t^2 - 0 t + 0 = 0 \implies t^2 = 0 \implies t = 0
So (x,y)=(0,0)(x, y) = (0, 0)

- If S=1S = 1:
P=1212=0 P = \frac{1^2 - 1}{2} = 0
So xx and yy are roots of t2t=0    t(t1)=0t^2 - t = 0 \implies t (t - 1) = 0
So (x,y)=(0,1)(x, y) = (0, 1) and (1,0)(1, 0)

- If S=2S = 2:
P=2222=422=1 P = \frac{2^2 - 2}{2} = \frac{4 - 2}{2} = 1
So xx and yy are roots of t22t+1=0    (t1)2=0    t=1t^2 - 2 t + 1 = 0 \implies (t - 1)^2 = 0 \implies t = 1
So (x,y)=(1,1)(x, y) = (1, 1)

Therefore, all solutions are:
(x,y)=(0,0), (0,1), (1,0), (1,1)(x, y) = (0, 0),\ (0, 1),\ (1, 0),\ (1, 1)

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.