What is the meaning of co-interior angles?

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Detailed Solution

Co-interior angles are the pair of interior angles on the same side of the transversal when two lines are crossed by that transversal. If the lines are parallel these angles add up to 180 degrees.

When two lines are cut by a third line (called the transversal), several angles are formed. Among them are the interior angles on the same side of the transversal between the two lines. Those are the co-interior angles.
If the two original lines are parallel, then each pair of co-interior angles are supplementary (their sum = 180°). This is a well-known theorem in Euclidean geometry.
Why the term “co-interior”: “Co” means together/with, “interior” refers to the region between the two lines. So co-interior refers to angles together inside that region, on the same side of the transversal.

Term	Definition
Co-interior angles	Interior angles on same side of transversal between two lines
Property (parallel lines)	Sum equals 180° if lines are parallel

Example: If line l and line m are parallel, and line t is a transversal, then angle A (interior on one side) + angle B (interior on the same side) = 180°.

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