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Co-interior Angles

Part of the Angles section of Coddy's Geometry journey. Lesson 34 of 62.

Co-interior angles lie between two lines and on the same side of a transversal, forming a C or U shape. When the lines are parallel, these angles add to 180°. They need not be equal, although both can be 90°. To see why they add to 180°, use an equal alternate angle to form a linear pair with the other co-interior angle.

$Co-interior angles between two parallel lines: 125 degrees and w degrees$

In the figure, 125 + w = 180, so w = 55. For parallel lines, corresponding angles and alternate angles are equal, while co-interior angles add to 180°.

Challenge

Medium

The marked angles are co-interior: (2x + 20)° and 60°.

$Co-interior angles between two parallel lines: 2x plus 20 degrees and 60 degrees$

Find x.

Try it yourself

(2x + 20) + 60 = 180

Solve for x

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Angles

1Meet the Angle

Your First Angle Naming Angles Adding Adjacent Angles Degrees and the Full Turn Acute, Right, Obtuse Recap: Adjacent Angles

4Vertical Angles

Linear Pairs Vertical Angles Are Equal Solving with Verticals All Four Angles Recap: At the Crossing

7Crossing Parallel Lines

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

10Isosceles Triangles

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

2Straight Lines and Supplements

Angles on a Line Supplementary Angles Three Angles on a Line 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

8Transversal Problems

Both Angles Labeled Verticals Meet Parallels From Figure to Equation Alternate with Algebra 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

Complementary Angles Finding the Complement Perpendicular Lines 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