wunder beta📐 Working with Angles
Name angles, measure them with a protractor, and use powerful angle facts — straight lines, points, and crossing lines — to hunt down missing angles.
What you’ll learn
- Naming and Measuring AnglesName angles by size and measure them accurately with a protractor.An angle measures an amount of turn between two lines meeting at a vertex, counted in degrees. Angles are named by size: acute (< 90°), right (90°), obtuse (90°–180°), straight (180°), and reflex (> 180°). To measure one, place the protractor's centre on the vertex, line one arm up with zero, and read where the other arm crosses the scale.
- Angle Facts on Lines and PointsApply the key angle facts for straight lines, points, and crossing lines.Three facts solve most angle problems: angles on a straight line add up to 180°, angles all the way around a point add up to 360°, and vertically opposite angles formed by two crossing lines are equal. Recognising which fact applies is the key to setting up the right calculation.
- Finding Missing AnglesCombine angle facts to calculate unknown angles, including inside triangles.To find a missing angle, first identify which fact applies, then set up and solve the arithmetic. Angles on a line total 180°, angles around a point total 360°, angles in a triangle total 180°, and angles in a quadrilateral total 360°. Subtract the known angles from the total to reveal the missing one, and always check that the values add back correctly.
Questions this course answers
In what units do we measure angles?
Angles measure an amount of turn, and that turn is measured in degrees, shown with the ° symbol.
An angle of 130° is which kind of angle?
130° is between 90° and 180°, which makes it an obtuse angle.
When measuring with a protractor, where do you place its centre?
The protractor's centre goes on the vertex, the point where the two arms of the angle meet.
Two angles sit next to each other on a straight line. What must they add up to?
Angles on a straight line always add up to 180°, because a straight line is a half turn.
When two straight lines cross, the angles opposite each other are...
Vertically opposite angles — the pair across the X from each other — are always equal.
Angles fit together all the way around a point. Their total is...
Angles around a point make a full turn, so they add up to 360°.
Grounded in trusted sources
- Khan Academy — angles
- National Council of Teachers of Mathematics
- BBC Bitesize — angle facts
- Britannica — angle (geometry)
Every Wunder lesson is built from real, reputable sources — never invented.
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