Base Angle Theorem
From AoPSWiki
The Base Angle Theorem states that in an isosceles triangle, the angles opposite the congruent sides are congruent.
Proof
Since the triangle only has three sides, the two congruent sides must be adjacent. Let them meet at vertex 
.
Now we draw altitude 
to 
. From the Pythagorean Theorem, 
, and thus 
is congruent to 
, and 
. 
