Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it India

Problem:
Let α\alpha and β\beta be positive integers such that
43197<αβ<1777 \frac{43}{197}<\frac{\alpha}{\beta}<\frac{17}{77}

Find the minimum possible value of β\beta.

Solution

Solution:
We have
7717<βα<19743 \frac{77}{17}<\frac{\beta}{\alpha}<\frac{197}{43}
That is,
4+917<βα<4+2543 4+\frac{9}{17}<\frac{\beta}{\alpha}<4+\frac{25}{43}
Thus 4<βα<54<\frac{\beta}{\alpha}<5. Since α\alpha and β\beta are positive integers, we may write β=4α+x\beta=4 \alpha+x, where 0<x<α0<x<\alpha. Now we get
4+917<4+xα<4+2543 4+\frac{9}{17}<4+\frac{x}{\alpha}<4+\frac{25}{43}
So 917<xα<2543\frac{9}{17}<\frac{x}{\alpha}<\frac{25}{43}; that is, 43x25<α<17x9\frac{43 x}{25}<\alpha<\frac{17 x}{9}.
We find the smallest value of xx for which α\alpha becomes a well-defined integer. For x=1,2,3x=1,2,3 the bounds of α\alpha are respectively (11825,189),(31125,379),(549,523)\left(1 \frac{18}{25}, 1 \frac{8}{9}\right),\left(3 \frac{11}{25}, 3 \frac{7}{9}\right),\left(5 \frac{4}{9}, 5 \frac{2}{3}\right). None of these pairs contain an integer between them.
For x=4x=4, we have 43x25=61225\frac{43 x}{25}=6 \frac{12}{25} and 17x9=759\frac{17 x}{9}=7 \frac{5}{9}. Hence, in this case α=7\alpha=7, and β=4α+x=28+4=32\beta=4 \alpha+x=28+4=32.
This is also the least possible value, because, if x5x \geq 5, then α>43x25435>8\alpha>\frac{43 x}{25} \geq \frac{43}{5}>8, and so β>37\beta>37.
Hence the minimum possible value of β\beta is 32.

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