Geometry · Parallel Lines & Angle Pairs
Co-Interior Angles
HSG-CO.C.9
Definition
Same-side interior angles between two parallel lines that sum to 180 degrees.
Teach it
Where it is taught, with guided notes, practice, homework, an exit ticket and an interactive Teacher Deck.
Examples
- 120 + 60 = 180 degrees for a pair of co-interior angles on parallel lines.
- Co-interior angles sit between the lines, on the same side of the transversal.
Key rule
Co-interior angles are supplementary when the lines are parallel.
Memory hook
Same side, inside — they add to 180 degrees.