Base Angle Theorem
The Hinge Theorem states that in an isosceles triangle, the measures of the angles opposite the equal-measuring sides are equal.
Proof
Since the triangle only has three sides, the two equal-measuring sides must be adjacent. Let them meet at vertex 
. Now we draw height 
to 
. From the Pythagorean Theorem, 
, and thus 
is similar to 
, and 
.
