Tell them apart

01.07 Line and Angle Proofs — the 4 pairs that get mixed up

Most mix-ups happen when two words sit next to each other. These are the 4 pairs worth practicing apart. The 4 marked most confused are the ones the course itself flags, or that sit closest together.

Cover the right-hand column with your hand, read the left definition, and say which term it is out loud before you look. Then swap sides.

most confused
Corresponding

Same side of the path, same slot. The F. Congruent if parallel.

vs
Alternate interior

Inside, opposite sides. The Z. Also congruent if parallel.

How to tellBoth say congruent. Ask where they sit, not whether they match.
most confused
Same-side interior

Inside, same side. The C. Add to 180° if parallel.

vs
Alternate interior

Inside, opposite sides. The Z. Congruent if parallel.

How to tellBoth are interior. Same side vs opposite side is the whole difference.
most confused
Vertical

Across the X. Nonadjacent. Congruent.

vs
Linear pair

Next to each other on a straight line. Adjacent and supplementary.

How to tellAcross vs beside. Matching vs adding to 180°.
most confused
Substitution Property of Equality

Replace a name with a number you already know.

vs
Addition Property of Equality

Add the same amount to both sides of an equation.

How to tellSame 60° can appear in two choices. Swapping a name for 60° is substitution, not addition.