8th Grade · Geometry
Co-Interior Angles
8.G.A.5
Definition
A pair of angles between two lines and on the same side of a transversal. When the lines are parallel, these angles are supplementary, summing to 180 degrees.
Examples
- If one co-interior angle is 110°, the other is 70° (110+70=180).
- Also called same-side interior angles.
Key rule
Co-interior angles are supplementary — they add to 180°.
Memory hook
Same side, inside the lines, add to 180°.