§3.1 Triangles

1. §3.1 Triangles The student will learn about: congruent triangles, proof of congruency, and some special triangles. 1

2. §3.1 Congruent Triangles The topic of congruent triangles is perhaps the most used and important in plane geometry. More theorems are proven using congruent triangles than any other method. 2

3. Triangle Definition A triangle is the union of three segments (called its sides), whose end points (called its vertices) are taken, in pairs, from a set of three noncollinear points. Thus, if the vertices of a triangle are A, B, and C, then its sides are , and , and the triangle is then the set defined by , denoted ΔABC. The angles of ΔABC are A  BAC, B  ABC, and C  ACB. 3

4. Euclid Euclid’s idea of congruency involved the act of placing one triangle precisely on top of another. This has been called superposition. 4

5. CONGRUENCY Definitions Angles are congruent if they have the same measure. Segments are congruent if they have the same length. 5

6. Definition Two triangles are congruent iff the six parts of one triangle are congruent to the corresponding six parts of the other triangle. One concern should be how much of this information do we really need to know in order to prove two triangles congruent. Congruency is an equivalence relation – reflexive, symmetric, and transitive. 6

7. We know that corresponding parts of congruent triangles are congruent. We abbreviate this fact as CPCTC and find it quite useful in proofs. Properties of Congruent Triangles 7

8. Important Note When we write Δ ABC  Δ DEF we are implying the following: A  D B  E C  F AB  DE BC  EF AC  DF Order in the statement, Δ ABC  Δ DEF, is important. 8

9. We Will Use CPCTE To Establish Three Types of Conclusions 1. Proving triangles congruent, like Δ ABC and Δ DEF. 2. Proving corresponding parts of congruent triangles congruent, like • Establishing a further relationship, like • A  B. 9

10. Some Postulate Postulate 12. The SAS Postulate Every SAS correspondence is a congruency. Postulate 13. The ASA Postulate Every ASA correspondence is a congruency. Postulate 12. The SSS Postulate Every SSS correspondence is a congruency. 10

11. Marking Drawings B D A C AB  CD A  C AC  BD CBD   BCA AC  BD 11

12. Suggestions for proofs that involve congruent triangles: Mark the figures systematically, using: A.  A square in the opening of a right triangle; Mark the figures systematically, using: A.  A square in the opening of a right triangle; B.   The same number of dashes on congruent sides; And. Mark the figures systematically, using: A.  A square in the opening of a right triangle; B.   The same number of dashes on congruent sides; And. C.   The same number of arcs on congruent angles. Mark the figures systematically, using: A.  A square in the opening of a right triangle; B.   The same number of dashes on congruent sides; And. C.   The same number of arcs on congruent angles. D. Use coloring to accomplish the above. Mark the figures systematically, using: A.  A square in the opening of a right triangle; B.   The same number of dashes on congruent sides; And. C.   The same number of arcs on congruent angles. D. Use coloring to accomplish the above. F. If the triangles overlap, draw them separately. 12

13. Example Proof H A F R B Given: AR and BH bisect each other at F Prove: AB  RH 13