Which statement is an example of a mathematical theorem?

• A. The angles in a triangle add up to 180 degrees
• B. The product of two negatives is negative
• C. The sum of digits of 11 is 2
• D. A number is equal to itself

Answer: (A)

A statement that can be proven from basic assumptions is a theorem. The statement that the angles in every triangle add up to 180 degrees is such a universal result: it holds for any triangle and has a standard proof, for example by drawing a parallel line to the base and using corresponding angles, or by the exterior angle theorem. The other options aren’t theorems in the same sense: one is actually false (negative times negative gives a negative), another is just a calculation about a specific number (the sum of the digits of 11), and the last is a basic identity property treated as an axiom rather than a proven result. So the angle-sum statement is the correct example of a mathematical theorem.