Same-Side Exterior Angles
Given: a||b
Prove: ∠1 and ∠8 are supplementary angles
It's given that a||b. Since corresponding angles are congruent, ∠1 ≅ ∠5.
∠5 and ∠8 form a straight line. Thus, ∠5 and ∠8 are supplementary
angles. Therefore, by substitution, ∠1 and ∠8 are supplementary
angles.
If two parallel lines
are cut by a transversal,
the exterior angles on the same side
are supplementary.
© Jory Stark