Maths Olympiad Prep

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Number theory Difficulty 5.1 AIME, harder Find the answer

Example 4 Find the solution to 117x1+21x2=38117 x_{1}+21 x_{2}=38.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Solving from the original equation, we get
x2=121(117x1+38)=6x1+2+121(9x14)x3=121(9x14)Zx1=19(21x3+4)=2x3+19(3x3+4)x4=19(3x3+4)Zx3=13(9x44)=3x4113\begin{array}{l} x_{2}=\frac{1}{21}\left(-117 x_{1}+38\right)=-6 x_{1}+2+\frac{1}{21}\left(9 x_{1}-4\right) \\ x_{3}=\frac{1}{21}\left(9 x_{1}-4\right) \in \boldsymbol{Z} \\ x_{1}=\frac{1}{9}\left(21 x_{3}+4\right)=2 x_{3}+\frac{1}{9}\left(3 x_{3}+4\right) \\ x_{4}=\frac{1}{9}\left(3 x_{3}+4\right) \in \boldsymbol{Z} \\ x_{3}=\frac{1}{3}\left(9 x_{4}-4\right)=3 x_{4}-1-\frac{1}{3} \end{array}

The last equation indicates: x3,x4x_{3}, x_{4} cannot both be integers, so the indeterminate equation has no solution.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.