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HongKong -23 IMO Selection Tests, 2022

Geometry Difficulty 7.5 National Olympiad, round 2 Prove it Hong Kong

In △ABC\triangle ABC, AB<ACAB < AC. The internal bisector of ∠BAC\angle BAC meets BCBC at DD, while the external bisector of ∠BAC\angle BAC meets CBCB produced at EE. If EB=2022EB = 2022 and the lengths of BDBD and DCDC are integers, how many possible lengths of BDBD are there?

Solution

Let the lengths of BDBD and DCDC be xx and yy respectively. By the angle bisector theorem, we have xy=ABAC=EBEC\frac{x}{y} = \frac{AB}{AC} = \frac{EB}{EC}, i.e.
xy=20222022+x+y \frac{x}{y} = \frac{2022}{2022 + x + y}

Figure 1

y=x2+2022x2022−x=−x−4044+2022⋅40442022−x y = \frac{x^2 + 2022x}{2022 - x} = -x - 4044 + \frac{2022 \cdot 4044}{2022 - x}
Hence we must choose positive integers xx for which 2022−x2022 - x is a positive integer dividing 2022⋅40442022 \cdot 4044 (note that every such xx will result in a positive integer value of yy and the existence of such a figure satisfying all the conditions). As 2022⋅4044=23⋅32⋅33722022 \cdot 4044 = 2^3 \cdot 3^2 \cdot 337^2, its only positive factors that are less than 20222022 are the 12 positive factors of 23⋅322^3 \cdot 3^2 as well as 337,337⋅2,337⋅3337, 337 \cdot 2, 337 \cdot 3 and 337⋅4337 \cdot 4. Hence there are altogether 16 such values of 2022−x2022 - x, and they correspond to 16 possible values of xx.

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