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Algebra Difficulty 4.8 AIME Prove it United States

Problem:
Suppose a,b,ca, b, c are rational numbers such that
(a2+1)3=b+1(b2+1)3=c+1(c2+1)3=a+1 \begin{aligned} & \left(a^{2}+1\right)^{3}=b+1 \\ & \left(b^{2}+1\right)^{3}=c+1 \\ & \left(c^{2}+1\right)^{3}=a+1 \end{aligned}
Prove that a=b=c=0a=b=c=0.

Solution

Solution:
We have that b=(a2+1)31b=\left(a^{2}+1\right)^{3}-1, c=(b2+1)31c=\left(b^{2}+1\right)^{3}-1, and a=(c2+1)31a=\left(c^{2}+1\right)^{3}-1. By direct substitution we derive that aa satisfies the following polynomial equation of degree 216:
(((((a2+1)31)2+1)31)2+1)3(a+1)=0 \left(\left(\left(\left(\left(a^{2}+1\right)^{3}-1\right)^{2}+1\right)^{3}-1\right)^{2}+1\right)^{3}-(a+1)=0
We observe that the polynomial can be rewritten as
a216+c215a215++c2a2a=0 a^{216}+c_{215} a^{215}+\cdots+c_{2} a^{2}-a=0
for some integers c2,,c215c_{2}, \ldots, c_{215}. Hence by the Rational Root Theorem, if a0a \neq 0 then it follows that a=±1a= \pm 1. So a{1,0,1}a \in\{-1,0,1\}. Similarly, b,c{1,0,1}b, c \in\{-1,0,1\} as well.

But if a=±1a= \pm 1, then we have b=(1+1)31=7b=(1+1)^{3}-1=7, which is impossible. Hence only a=0a=0 can occur. Thus a=b=c=0a=b=c=0.

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