A.3 How do they differ?

Angles

Lines r and s are parallel. Alex finds the relationship between ∠3 and ∠5.

m∠3 = 70°
Alex's "Corresponding & Supplementary Angles" Method
∠3 and ∠7 are corresponding angles. Since ∠3 = 70°, then ∠7 is the same.
Because ∠7 and ∠5 add to 180°,
I know that
180° - 70° = ∠5.
∠3 and ∠5 add to 180°.
70° = 70°
180° − 70° = 110°
70° + 110° = 180°
m∠3 + m∠5 = 180°