Q:
# What Is a Co-Interior Angle?

A co-interior angle is formed when two lines are intersected by a third line in two distinct points. The four angles that lie on the inside of the two lines are called interior angles.

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Johner Images
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The third line that intersects the two lines is called the transversal. Co-interior angles always lie on the same side of the transversal, never on opposite sides. The sum of a pair of co-interior angles is always equal to 180 degrees if the two lines are parallel to each other. When the two lines are not parallel, the sum of the two co-interior angles would not be exactly 180 degrees. This is an important axiom in geometry.

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