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Example:
We know they are congruent without measuring the angles because of SSS.
Discuss each of the others SAS and ASA. Discuss why AAA and SSA don’t work as
congruence shortcuts. Discuss how AAS is just ASA because if 2 angles in a triangle are
congruent then third must be as well.
Lesson joke: If it smells like SSA then it is SSA.
Lesson Question: Why are the base angles of an isosceles triangle congruent?
Prove: Why the base angles of an isosceles triangle are congruent.
B
A
C
Demo how to write a proof
1. Use sketchpad as an inductive proof. (10 minutes, this is optional)
2. Locking a triangle with SAS postulate
3. Have the students vote on SSS, ASA, SSA, AAA. (15 minutes)
4. Practice problems from page 124 classroom exercises.
Classwork/Homework: Read section 4 -1 in your book do problems 5-16. Write a
paragraph proof that shows why the base angles of an isosceles triangle are congruent.
The paragraph should look something like this:
B
A
E
C
Triangle ABC is known to be isosceles. This tells us that AB  BC .
Next we bisect angle ABC forming two triangles. If we can prove these triangles
are congruent then we can show that the base angles are also congruent.
First show angle CBE congruent to angle ABE because we bisected them then
they share BE therefore through SAS they are congruent and by extension angles
A and C are also congruent. Have students write a paragraph proof and share it
with their group then have a couple students write their proof on an overhead and
present to the class.
Plan for independent practice… Classwork or homework: Page 137 # 1-8. Page 124125 written exercises # 1 – 15.