Maths Olympiad Prep

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Algebra Difficulty 5.2 AIME, harder Prove it Philippines

Problem:
Let aa, bb and cc be positive integers such that a2013+bb2013+c\frac{a \sqrt{2013}+b}{b \sqrt{2013}+c} is a rational number. Show that a2+b2+c2a+b+c\frac{a^{2}+b^{2}+c^{2}}{a+b+c} and a32b3+c3a+b+c\frac{a^{3}-2 b^{3}+c^{3}}{a+b+c} are both integers.

Solution

Solution:
By rationalizing the denominator, a2013+bb2013+c=2013abbc+2013(b2ac)2013b2c2\frac{a \sqrt{2013}+b}{b \sqrt{2013}+c}=\frac{2013 a b-b c+\sqrt{2013}\left(b^{2}-a c\right)}{2013 b^{2}-c^{2}}. Since this is rational, then b2ac=0b^{2}-a c=0. Consequently,
a2+b2+c2=a2+ac+c2=(a+c)2ac=(a+c)2b2=(ab+c)(a+b+c) \begin{aligned} a^{2}+b^{2}+c^{2} & =a^{2}+a c+c^{2}=(a+c)^{2}-a c=(a+c)^{2}-b^{2} \\ & =(a-b+c)(a+b+c) \end{aligned}
and
a32b3+c3=a3+b3+c33b3=a3+b3+c33abc=(a+b+c)(a2+b2+c2abbcca) \begin{aligned} a^{3}-2 b^{3}+c^{3} & =a^{3}+b^{3}+c^{3}-3 b^{3}=a^{3}+b^{3}+c^{3}-3 a b c \\ & =(a+b+c)\left(a^{2}+b^{2}+c^{2}-a b-b c-c a\right) \end{aligned}
Therefore,
a2+b2+c2a+b+c=ab+c and a32b3+c3a+b+c=a2+b2+c2abbcca \frac{a^{2}+b^{2}+c^{2}}{a+b+c}=a-b+c \quad \text{ and } \quad \frac{a^{3}-2 b^{3}+c^{3}}{a+b+c}=a^{2}+b^{2}+c^{2}-a b-b c-c a
are integers.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.