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The Square Mark

Part of the Angles section of Coddy's Geometry journey. Lesson 8 of 59.

A right angle often carries no number at all, only the small square in its corner. That square is a measure like any label: it says 90°. When you need the angle's size, you read it off the square.

$A straight line with three angles: 35 degrees, a right angle marked with a square, and 55 degrees$

Here three angles fill the straight line PQ. The middle one has the square, so it is 90°, and the three add up to 180°: 35 + 90 + 55 = 180. If the 55° were missing, you would first say the middle angle is 90°, then take 35 and 90 away from 180.

That is two steps on the figure. Tap the square angle and pick Right angle: its 90° appears. Now the angle you want has every neighbour known, and Angles on a straight line finishes it.

Challenge

Two rays stand on the straight line PQ, and one of the three angles carries a square. Find the angle marked ?: give every step its rule, 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 Angles Co-interior Angles Which 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 MarkComplementary 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