Geometry · Quadrilaterals · Triangle inequalities · Side angle relations · Hinge theorem · Configuration analysis
A quadrilateral \(ABCD\) satisfies
\[ AD = BC \qquad \text{and} \qquad \angle DAB > \angle ABC . \]
Prove that then \(\angle BCD > \angle CDA\).
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Serbian Regional Competition (Okruzno takmicenje) 1997, high school grade I, problem 5. The 1997 regional paper was not split into categories. Organized by the Mathematical Society of Serbia (DMS).