The Isoceles Triangle Theorem

This lesson is from pages 140-143 of Jacob's Geometry

Base Angle Theorem;
(Isosceles Triangle) If two sides of a triangle are congruent, the angles opposite these sides are congruent.

Base Angle Converse
(Isosceles Triangle) If two angles of a triangle are congruent, the sides opposite these angles are congruent.

A corollary is a theorem that can be easily proved from another theorem.

It follows that if the theorems above are true, then it is also true that all the angles in an equilateral triangle are also equal.  (And vice versa) -- if each angle in a triangle is 60 degrees, then it is an equilateral triangle.

Try:
This quiz on equilateral and isosceles triangles

DO:

Set I, 1-15, 21-26
Set II, #29