The diagram you've uploaded depicts an isosceles triangle with two bold equal sides and two congruent angles opposite these sides. This illustration suggests the Isosceles Triangle Theorem, which states:

If two sides of a triangle are congruent, then the angles opposite those sides are congruent.

In this diagram, the bold lines represent the congruent sides of the triangle, which form the basis of the theorem's hypothesis. Thus, the answer to your question is likely:

Isosceles Triangle Theorem

Would you like further details or have any questions?

Here are five related questions you might find interesting:

1. What is the Converse of the Isosceles Triangle Theorem?
2. How do Corollaries 1 and 2 of the Isosceles Triangle Theorem extend the main theorem?
3. How does CPCTC (Corresponding Parts of Congruent Triangles are Congruent) relate to triangle proofs?
4. How is the Isosceles Triangle Theorem useful in coordinate geometry?
5. What are the applications of the Isosceles Triangle Theorem in real-world problems?

Tip: In geometry, identifying congruent angles or sides early in a problem can often simplify finding unknown values.