1. The problem asks to identify the theorem used to obtain the third reason in a given context. 2. Typically, in geometry or algebra proofs, reasons correspond to theorems or properties applied to justify steps. 3. Without the original problem context, a common approach is to consider standard theorems such as the Pythagorean theorem, Triangle Sum theorem, or properties of equality. 4. If the third reason involves equality or congruence, it might be the Transitive Property, Substitution, or a specific theorem like SAS, ASA, or SSS in geometry. 5. To be precise, please provide the original problem or the first two reasons so the third reason's theorem can be identified accurately.