Euclid's Elements Book 1, Proposition 21

Project Euclid presents Euclid's Elements, Book 1, Proposition 21 "If from the ends of one of the sides of a triangle two straight lines are constructed meeting within the triangle, then the sum of the straight lines so constructed is less than the sum of the remaining two sides of the triangle, but the constructed straight lines contain a greater angle than the angle contained by the remaining two sides." Presented by Anthony Risolio, St. John's College, Class of 2015