Geometry

Question Analysis:
This tests logical structure of proof by contradiction, especially the assumption phase and contradiction derivation.
The main focus is on recognizing what must be assumed and how a contradiction emerges.

Key Concept Explanation:
To prove "If A, then B" by contradiction:
Assume A is true
Assume B is false (i.e., opposite of conclusion)
Reach contradiction

Step-by-Step Solution:
A is true: two angles are equal ✅
Contradict the conclusion: assume opposite sides are not equal ✅
Use triangle properties (e.g., converse of Isosceles Triangle Theorem)
Derive contradiction ⇒ B must be true ⇒ proof complete

Option Analysis:
A. ✅ You must assume the hypothesis is true in contradiction
B. ❌ Assuming equal sides supports the conclusion — not a contradiction
C. ✅ You ass...