"Things which are equal to the same thing are also equal to one another."

"To bisect a given rectilinear angle."

"A plane angle is the inclination of the lines to one another, when two lines meet one another, but are not in the same straight line."

"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."

"A quantity is said to be a part of a quantity, the less of the greater, when it measures the greater."