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VN TST 2025 Question 1

VMO · VN TST 2025

Question 1

Find all functions $f:\mathbb Q_{>0}\to\mathbb Q_{>0}$ such that, for every $x,y\in\mathbb Q_{>0}$,

$$\frac{f(x)f(y)}{f(xy)}=\frac{(\sqrt{f(x)}+\sqrt{f(y)})^2}{f(x+y)}.$$

Question point values reproduce the supplied source labels; no automatic proof grading is implied.