Angle facts and parallel lines – Year 9 maths lesson
Lesson 11 of 38 in the Year 9 maths course
UK version. UK school years, UK spelling and pounds (£). · US Math Department
A complete 45-minute lesson for Year 9 (ages 13–14) that runs on this page – at school or at home. Every answer is marked straight away, with a worked explanation.
- Time
- 45 minutes + 20 min homework
- Year
- Year 9 (ages 13–14)
- You need
- Paper, a pencil and this page
- Marked
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know this topic yourself. The lesson explains each step on screen, one tidy step at a time. Every answer is marked straight away with a worked explanation, so you never have to mark anything.
- Read what your child will learn and the big ideas together.
- Press Start the lesson below and sit with your child for the teaching and the worked examples (about 19 minutes).
- Let them do the practice on their own. They choose Getting started, On track or Challenge – start on On track and move up after a few right in a row.
- Finish with the exit ticket and the talk question: “Why do co-interior angles add up to 180°?”
- Later that day or the next day: the homework (20 minutes), then a practice test.
Watch out for
- Measuring with a protractor when the diagram is not to scale. Use angle facts and write the reason.
- Mixing up alternate and co-interior angles. Z shape: equal. C shape: add to 180°.
Tip for home: Find parallel lines and crossing lines in windows, railings and roads.
What your child will learn
We are learning to use angle facts and angles in parallel lines to find missing angles, giving reasons.
Success looks like
- I can use basic angle facts.
- I can use angles in parallel lines.
- I can give reasons for each step.
Key words
- vertically opposite
- equal angles opposite each other where two lines cross
- alternate angles
- equal angles in a Z shape
- corresponding angles
- equal angles in an F shape
- co-interior angles
- angles in a C shape that add to 180°
What to have ready
- Paper and a pencil
- This page on a computer, tablet or phone
- For the hands-on part: Draw two parallel lines and a crossing line, measure all eight angles and spot which are equal.
The big ideas (for the adult)
Angles on a straight line add up to 180°; angles around a point add up to 360°; vertically opposite angles are equal.
Angles in a triangle add up to 180°. The two base angles of an isosceles triangle are equal.
Where a line crosses parallel lines: alternate angles are equal, corresponding angles are equal, and co-interior angles add up to 180°.
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter: quick recall | 5 min | Marked by Classfern |
| Teaching input | 10 min | Not assessed, self-checked |
| Worked examples and guided practice | 9 min | Not assessed, marked by Classfern |
| Independent practice | 15 min | Marked by Classfern |
| Plenary and exit ticket | 6 min | Marked by Classfern, self-checked |
Starter, "your turn", practice and exit ticket are marked by Classfern. Worked examples and teaching are not assessed. The hands-on task and the reflection are self-assessed. Explanations in talk are judged by the parent and not recorded.
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser.
Homework and practice at home
20 minutes: 10 practice questions, 3 multiple-choice questions and 2 to remember from earlier work, then a short reflection. New questions every time, all marked instantly.
More practice
More Year 9 maths lessons
- Prime factors, HCF and LCM
- Rounding and estimation
- Bounds and error intervals
- Standard form
- Laws of indices
- Powers and roots
- Units and measures
- Area of triangles, parallelograms and trapeziums
- Circles: circumference, area, arcs and sectors
- Volume and surface area
- Angles in polygons
- Pythagoras' theorem
- Trigonometry: SOH CAH TOA
- Transformations
- Similarity and congruence
- Quadrilaterals and symmetry
- Coordinate geometry
- 3-D shapes, plans and elevations
- Quartiles, box plots and spread
- Pie charts and time series graphs
- Sampling
- Relative frequency and expected outcomes
- Venn diagrams and conditional probability
- Algebraic notation
- Substitution
- Simplifying expressions
- Expanding brackets
- Solving linear equations
- Inequalities
- Rearranging formulae
- Sequences and the nth term
- Straight-line graphs
- Graphs of functions
- Ratio
- Percentages
- Speed, density and pressure
- Scale drawings and maps
Curriculum
National Curriculum for England, Year 9: “derive and use the sum of angles in a triangle and use it to deduce the angle sum in any polygon, and to derive properties of regular polygons”
National Curriculum for England, Year 9: “understand and use the relationship between parallel lines and alternate and corresponding angles”
Contains public sector information licensed under the Open Government Licence v3.0.
Our lessons and questions are our own. They are not written or endorsed by any government department, testing agency or exam board.