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

m∠3 = 70°

∠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.

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°