Co-interior angles

I can recognise that a pair of parallel lines traversed by a straight line produces supplementary co-interior angles.

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Co-interior angles

I can recognise that a pair of parallel lines traversed by a straight line produces supplementary co-interior angles.

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Key learning points

Co-interior angles can be identified in an arrangement of 3 line segments.
If two of the lines are parallel then these angles are supplementary.
Knowledge of co-interior angles can be used to find unknown angles.
Knowledge of co-interior, corresponding and alternate angles can be used to find unknown angles.

Keywords

Co-interior - Co-interior angles are on the same side of the transversal line and in between the two other lines.

Common misconception

Pupils may believe that the co-interior angles are equal, like alternate and corresponding angles.

Have pupils draw a pair of parallel lines and a transversal line themselves, identify the co-interior angles, are they equal?

It can be helpful to show the pupils multiple diagrams with co-interior, alternate and corresponding angles identified and have the pupils decide which of the three is being shown at that point.

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This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).