The corpus record
emitrigonos
forming half a triangle
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ἡμι-τρίγωνος · hēmi-trigōnos — LSJ
forming half a triangle, τρίγωνον, of the triangles made by drawing a perpendicular from an angle of an equilateral triangle, Speus. ap. Theol.Ar. 63: neut. as Subst., Ti.Locr. 98a, Simp. in Cael. 561.13.
No etymology authority pointer is recorded for this lemma yet — an honest gap, not an omission. The etymological dictionaries (Beekes, Chantraine, Frisk) are matched incrementally.