To distinguish between the statements:
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Statement #1: All triangles have three sides and three angles.
- This is a definition of a triangle, not a theorem. It defines what a triangle is.
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Statement #2: The interior angles of any triangle sum to 180 degrees.
- This is a theorem in geometry, as it can be proven based on the properties of triangles.
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Statement #3: \( a + b = b + a \)
- This is known as the commutative property of addition, which is a mathematical property rather than a theorem.
In conclusion: Statement #2 is the theorem.