“Things which are equal to the same thing are also equal to one another.” Equal Book:The Thirteen Books of Euclid's Elements Source: The Thirteen Books of Euclid's Elements
“In right-angled triangles the square on the side subtending the right angle is equal to the squares on the sides containing the right angle.” ScienceSidesEqualSquaresAngleGeometryContainingTriangles Book:Elements of Geometry, Containing the First Six Books of Euclid Source: Elements of Geometry, Containing the First Six Books of Euclid