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Angles on a Line

Part of the Angles section of Coddy's Geometry journey. Lesson 7 of 62.

A straight line makes a straight angle: a half turn, 180°. When a ray stands on the line, it splits that straight angle into two angles that sit side by side. Together the two still span the whole line, so they always add to 180°.

$Two angles on a straight line, 120 degrees and x degrees$

In the figure the two angles measure 120° and x°. Side by side on the line they make the full 180°, so 120 + x = 180. Take 120 from both sides: x = 60. The rule to keep: angles on a straight line add to 180°.

Challenge

Easy

In the figure, two angles sit together on a straight line. One measures (x + 30)°, the other 50°.

$Two angles on a straight line, x plus 30 degrees and 50 degrees$

Find x.

Try it yourself

(x + 30) + 50 = 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

2Straight Lines and Supplements

Angles on a LineSupplementary 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