$\overleftrightarrow{AB}$ and $\overleftrightarrow{CD}$ are parallel lines. The interior angles of a triangle sum to $180^\circ$. **Complete the equations to prove that alternate interior angles always have equal measure.**  ||| - | -: | - $\text{In terms of } \purple w \text{ and a number} \\\text{in degrees,}$ | $\green v=$ | [[☃ expression 2]] $\text{In terms of } \purple w \text{ and a number} \\\text{in degrees,}$ | $\pink u=$ | [[☃ expression 3]] | $\text{Combining these equations to } \\ \text{get } \pink u \text{ in terms of } \green v,$ | $\pink u=$ | [[☃ expression 4]]