1. The problem asks: What is the reason for 6? CPCTC? 2. CPCTC stands for "Corresponding Parts of Congruent Triangles are Congruent." It is a theorem used in geometry to justify that if two triangles are proven congruent, then all their corresponding sides and angles are equal. 3. Step 6 in a geometric proof often states that a particular pair of sides or angles are equal because of CPCTC. 4. This means that after proving two triangles congruent (using criteria like SSS, SAS, ASA, AAS, or HL), we can conclude that their corresponding parts are equal. 5. Therefore, the reason for step 6 is that it relies on the CPCTC theorem to justify the equality of corresponding parts after establishing triangle congruence. 6. In summary, step 6 uses CPCTC to conclude that the specific parts mentioned are congruent because the triangles themselves are congruent.