3. A rectangle, the ratio of whose sides is 5, has the largest area among all rectangles whose vertices lie on the sides of a given rhombus, and whose sides are parallel to the diagonals of the rhombus. Find the acute angle of the rhombus.
3. $2 \operatorname{arctg} \frac{1}{5} \equiv \operatorname{arctg} \frac{5}{12}$.

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3. $2 \operatorname{arctan} \frac{1}{5} \equiv \operatorname{arctan} \frac{5}{12}$.