The missing reason in Huy's proof is the Transitive Property of Equality.
Here's the reasoning:
- Statement 1 establishes that \( m∠A + m∠C = 180° \).
- Statement 2 states that \( m∠A = m∠E \).
- Statement 3 uses substitution to show that \( m∠E + m∠C = 180° \), using the equality from Statement 2.
- Finally, Statement 4 concludes that \( ∠E \) and \( ∠C \) are supplementary based on their angle measures summing to 180°.
Thus, the reason for Statement 3 where substitution is made is based on the Transitive Property of Equality, whereby if \( m∠A = m∠E \), you can substitute \( m∠E \) for \( m∠A \) in the equation for the supplementary angles.