Angles on a straight line
If one angle is 70 degrees on a straight line, the other is 110 degrees.
Angles on a straight line add to 180 degrees.
Year 5
Identify angles at a point and one whole turn, on a straight line and at a point on a line, and use these facts to find missing angles
Use known angle facts to calculate missing angles on lines, around points and in turns.
Lesson overview
Geometry - properties of shape
Some angle totals are fixed. A straight line makes 180 degrees, a whole turn makes 360 degrees, and a right angle makes 90 degrees. We can use these facts to work out unknown angles.
Angles around a point
If three angles around a point total 250 degrees, the missing angle is 110 degrees.
Angles around a point add to 360 degrees.
Use right angles
If a line meets another line at a right angle, the angle is 90 degrees.
Known right angles can help split a problem into smaller parts.
Worked example
Two angles on a straight line are 125 degrees and x. Find x.
- Angles on a straight line add to 180 degrees.
- Subtract the known angle from 180.
- 180 - 125 = 55.
- So x is 55 degrees.
Round 1: Learn it
Choose the correct total first: 90, 180 or 360 degrees.
Finish this round to unlock Round 2: Show you know it.
Question 1
What do angles on a straight line add to?
Question 2
If one angle on a straight line is 70 degrees, what is the other angle?
Question 3
What do angles around a point add to?
Question 4
Three angles around a point total 250 degrees. Find the missing angle.
Question 5
A right angle measures how many degrees?
Common mistakes
Watch for these as you work through the lesson.
- Using 180 degrees when the angles are around a point.
- Forgetting to include all the known angles before subtracting.
- Mixing up right angles, straight lines and whole turns.
Related topics
These ideas connect closely to this lesson.
- Know angles are measured in degrees, estimate and compare acute, obtuse and reflex angles.
- Draw given angles, and measure them in degrees.