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

8. Important Note Order in the statement, Δ ABC  Δ DEF, is important.
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 A B C D AC  BD AC  BD CBD   BCA AB  CD A   C11

12. Suggestions for proofs that involve congruent triangles:
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.
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.
Mark the figures systematically, using:
A.  A square in the opening of a right triangle; 12

13. Example Proof Given: AR and BH bisect each other at F Prove: AB  RH B
Statement
Reason
1. AR and BH bisect each other.
Given
2. AF = FR and BF = FH
Definition of bisect.
3. AFB = RFH
Vertical Angle Theorem
4. ∆AFB = ∆RFH
ASA
5. AB = RH
CPCTE
6. QED 13

14. Definition – Angle Bisector
If D is in the interior of BAC, and BAD is congruent to DAC then bisects BAC, and is called the bisector of BAC. B D C A 14

15. Definition – Special Triangles
A triangle with two congruent sides is called isosceles. The remaining side is the base. The two angles that include the base are base angles. The angle opposite the base is the vertex angle.
A triangle whose three sides are congruent is called equilateral.
A triangle no two of whose sides are congruent is called scalene.
A triangle is equiangular if all three angles are congruent. 15

16. Theorem - Isosceles Triangle Theorem
The base angles of an Isosceles triangle are congruent.
Proof is a homework assignment. 16

17. Theorem – Converse of the Isosceles Triangle Theorem
If two angles of a triangle are congruent, then the sides opposite them are congruent.
Proof is a homework assignment. 17

18. Definition – Right Triangles
A triangle with one right angle is a right triangle.
Because two right triangles automatically have one angle congruent (the right angle), congruency of two right triangles reduces to two cases:
HA which is equivalent to ASA since all angles are known, and
HL which is equivalent to SSS since all three sides are know.
We are assuming knowledge of angle sums and Pythagoras. 18

19. Assignment: §3.1