"In isosceles triangles the angles at the base are equal to one another, and, if the equal straight lines be produced further, the angles under the base will be equal to one another."

"The square on the side subtending the right angle in right-angled triangles is equal to the squares on the sides containing the right angle."

"Parallelograms which are on the same base and in the same parallels are equal to one another."

"Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has only two of its sides equal, and a scalene triangle that which has its thre…"

"Sire, there is no royal road to geometry."