Euclid's Elements Book 1, Proposition 4

Project Euclid presents Euclid's Elements, Book 1, Proposition 4

"If two triangles have two sides equal to two sides respectively, and have the angles contained by the equal straight lines equal, then they also have the base equal to the base, the triangle equals the triangle, and the remaining angles equal the remaining angles respectively, namely those opposite the equal sides."

Presented by Daniel Sullivan, St. John's College, Class of 2015

[baxtersapper]

It would be cool if you did an explanation of the logic that goes behind assuming a straight line works how it does in Euclid.

[Aycore]

One simply must be rigorous when proving maths problems. Fair attempt though.