LESSON 3.10: ADDITIONAL TRIANGLE PROOF LESSON Ve have now seen all methods by which we will prove triangles congruent, i.e. SAS, ASA, SSS, AAS, and HL. We have also learned facts about parallel lines that can be used in proof as well as facts about isosceles triangles. The lesson today seeks to give more practice in the reasoning behind proof and the writing of proofs. Exercise #1: In the following diagram it is known that ED = FD, EB = FC , and DA bisects EF Prove: ABAD = ACAD (a) First, draw out a diagram to plan how the proof will work. Work B backwards by thinking about what you will need and what you know. Then sketch out a rough plan of how to attack this proof. A D F (b) Do the formal proof below