Mihova, Vesselka, and Julia Ninova. “DIRECT AND INDIRECT METHODS OF PROOF. THE LEHMUS-STEINER THEOREM”. Ann. Sofia Univ. Fac. Math. and Inf. 103 (December 12, 2016): 105–121. Accessed November 24, 2024. https://annual.uni-sofia.bg/index.php/fmi/article/view/59.