Answer:
Substitution
Explanation:
We are to complete the proof that:
m∠U + m∠Y + m∠UVY = 180° (without using the Triangle Angle Sum Theorem)
This is proven below given the following statements:
[tex]\begin{gathered} m\angle UVY+m\angle WVY=180^{\circ} \\ But:m\angle WVY=m\angle U+m\angle Y \\ \text{Substituting this into the initial formula, we have:} \\ \operatorname{\Rightarrow}m\operatorname{\angle}UVY+m\operatorname{\angle}U+m\operatorname{\angle}Y=180^{\operatorname{\circ}} \\ m\operatorname{\angle}U+m\operatorname{\angle}Y+m\operatorname{\angle}UVY=180^{\operatorname{\circ}} \\ \\ m\operatorname{\angle}U+m\operatorname{\angle}Y+m\operatorname{\angle}UVY=180^{\operatorname{\circ}}(Substitution) \end{gathered}[/tex]
Therefore, the answer is Substitution
