From: A scaffolding toolkit to foster argumentation and proofs in mathematics: some case studies
Erika’s free arguments | A B C D is a parallelogram because it has two parallel sides |
A E C D is a trapezoid because it has two parallel sides and two oblique sides. | |
Erika’s formal proof | A B C D is a parallelogram because it has opposite parallel sides |
A E C D is a trapezoid by Def. 2a |