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Co-interior Angles

Part of the Angles section of Coddy's Geometry journey. Lesson 31 of 59.

Two interior angles on the same side of the transversal are co-interior angles. They trace a C or a U. Unlike the other pairs, they need not be equal (only when both are 90°): with parallel lines they add up to 180°.

$Parallel lines and a transversal: co-interior angles of 120 and 60 degrees on the same side$

Here 120° and 60° sit between the parallels on the right of the transversal, and 120 + 60 = 180. The reason: the 60° angle is alternate to the angle next to the 120°, and those two neighbours sit on a straight line. So the 120° and the 60° fill a straight line between them.

On the board this is Co-interior angles. Mixing it up with the equal pairs is the classic mistake, so look at which side each angle is on before you pick.

Challenge

AB ∥ CD, cut by the transversal GH. Find the angle marked ?, then press Check.

Try it yourself

$Find the marked part of the figure$

Find the marked part of the figure

All lessons in Angles

1Meet the Angle

Your First Angle Angles on a Line Degrees and the Full Turn Across the Crossing

4Vertical Angles

Two Lines Cross Vertical Angles Are Equal Solving with Verticals All Four Angles Recap: At the Crossing

7Parallels and Transversals

Parallel Lines Corresponding Angles Alternate AnglesCo-interior AnglesWhich Rule Is It Recap: The Three Pairs

10Isosceles and Equilateral

Base Angles Are Equal Finding the Apex Which Angles Match Equilateral Angles Isosceles at Work Recap: Twin Corners

2Naming and Equations

Adding Adjacent Angles Write the Equation Recap: Along the Line

5Around a Point

Angles Around a Point The Missing Angle Equal Slices Four Angles, One Point Recap: The Full Turn

8Angle Chases

Verticals Meet Parallels Is There Enough?From Figure to Equation Both Angles Labeled Recap: Through the Strip

11The Exterior Angle

The Exterior Angle Using the Shortcut Two Roads, One Answer Exterior in the Wild Recap: Outside View

3Right Angles and Complements

The Square Mark Complementary Angles Perpendicular Lines Finding the Complement Equal Parts of a Corner Recap: The Right Corner

6The Angle Bisector

The Angle Bisector Bisected Expressions Bisecting a Straight Angle Recap: Split in Half

9Triangle Angle Sum

The 180 Rule Finding the Third Angle Two Unknown Angles Right Triangles Recap: The Budget

12Final Challenges

Final: The Angle Chase Final: Triangle Meets Line Final: The Full Round Final: Isosceles Chase Final: Master of Angles