Geodesic
Co možná nevíte o čtyřúhelníku
Rozhledy matematicko-fyzikální, Tome 85 (2010) no. 1, pp. 1-12.

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Many propositions about triangles are discussed in the school mathematics. There are formulas for computing the area of a triangle and relations between its sides and internal angles (Law of Sines, Law of Cosines). Using this knowledge, analogous formulas for quadrangles can be proved. The paper is not only the overview of the formulas, but the proofs are also included, as it is understanding of proofs that improves the skills necessary in geometry. 
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     author = {Hav{\'\i}\v{r}ov\'a, Barbora},
     title = {Co mo\v{z}n\'a nev{\'\i}te o \v{c}ty\v{r}\'uheln{\'\i}ku},
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Havířová, Barbora. Co možná nevíte o čtyřúhelníku. Rozhledy matematicko-fyzikální, Tome 85 (2010) no. 1, pp. 1-12. http://geodesic.mathdoc.fr/item/RMF_2010__85_1_a0/