This paper discusses geometric object called complete quadrilateral and some of its proprieties. Complete quadrilateral is configuration, represented by four lines. These four lines are unparallel and intersect two by two at six points. Main emphasis of this paper will be on ten theorems, first presented by Jacob Steiner in 1828. The paper formulates and proves all ten theorems. It also derives couple of corollaries of the theorems. Apart from Steiner’s theorems a few basic geometric results and concepts are discussed, such as the ones of pole and polar, inversion, circle of nine points, Menelaus’s theorem, which are needed for proving Steiner’s theorems.
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