Euclid — "The angles in the same segment are equal to one another."

"A straight line is a line which lies evenly with the points on itself."

"If a number be the least that is measured by any prime numbers, it will not be measured by any other prime number except those originally measuring it."

"If a straight line be cut into two equal parts and also into two unequal parts, the rectangle contained by the unequal parts together with the square on the line between the points of section is equal…"

"If a straight line touch a circle, and from the point of contact there be drawn across in the circle a straight line cutting the circle, the angles which it makes with the tangent will be equal to the…"

"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."