Measuring angles (age 9+)
Recognise angle measure as additive; find unknown angles by adding or subtracting on a diagram using equations with a symbol for the unknown
What a learner can do afterwards
- Two angles on a straight line are 65° and x°; find x = 115°
- An angle is decomposed into 35° and 40°; state the whole angle is 75°
- Solve: angles at a point are 120°, 90°, and x°; find x = 150°
The lesson
You already know that angles on a straight line add up to 180 degrees, and angles all the way around a point add up to 360 degrees. Now you can use those facts to find an angle you don't know yet. Give the missing angle a letter, like x, and write an equation. Then solve it.
Two angles sit next to each other on a straight line. One angle is 65°, and the other is x°. Since the two angles together make a straight line, write 65 + x = 180. Subtract 65 from both sides: x = 115°.
When an angle is missing, first find the total: 180° for a straight line, 360° for angles around a point. Then subtract the angles you already know. What's left over is your missing angle.
Give the unknown angle a letter, write an equation using the total, and solve for it.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.