Lesson 8A: Corresponding and Alternate angles
Yr 8 Objective: Identify Alternate and Corresponding angles
Pre-requisite Learning: Parallel and Perpendicular lines; sum of angles at a point, on a straight
line and in a triangle; vertically opposite angles.
Resources: OHP and OHTs 8A.1, 8A.2 & 8A.3, whiteboards (ideal but plain paper will do), tracing
paper
Vocabulary: Parallel, perpendicular, transversal, acute, obtuse, translate, rotate
Starter: Using whiteboards. Draw 2 different acute angles. Compare two different obtuse angles.
Draw 10 past 6 (or similar) - what sort of angle is it? Draw parallel lines – establish
language.
Main Activity:
Corresponding angles
Draw one pair of parallel lines and a transversal.
Establish language of corresponding angles.
Ask pupils to mark 2 acute angles that they think are
corresponding eg
Using tracing paper, trace the acute angle on the top parallel. Slide (translate) the tracing along the
transversal so that the angle sits on the bottom parallel.
What can we deduce? Is it true on everyone’s diagrams?
What other corresponding angles can we see?  mark them, trace and slide again. Emphasize
the generality of the result and ensure notes in books detail that corresponding angles between
parallel lines are equal.
Mini Plenary:
We have 8 angles in total, 4 of each type. When would all 8 angles be equal?
If I had another line parallel to the transversal, what shape could I have?
Alternate angles
Draw one pair of parallel lines and a transversal – mark and label two acute corresponding angles
as ‘c’. Mark x and y as shown (alternate angles)
c
Describe their relative positions – e.g. on different sides of a
x
transversal, between the parallels known as alternate
y
angles.
c
What do we notice?  they appear to be the same.
How can we be sure that they are the same?
Trace angle ‘x’. Rotate ‘x’ to vertically opposite position (mark x’) – pause to check, then
translate (slide) along the transversal to sit on the bottom parallel fitting onto y.
We know that x + c = 1800 and y + c = 1800 so ‘x’ must equal ‘y’. Write this down together.
Plenary:
Close your eyes.
What happens if the transversal line is rotated? What if the parallel lines get closer or further
apart? Visualise and describe. Are your corresponding angles still equal?
Are your alternate angles still equal?
Buckinghamshire KS3 Developing Geometrical Reasoning
Homework:
Use alternate angles to show that the opposite angles of a parallelogram are equal. See page
181 of the supplement of examples in the framework.
OR:
Refer back to 10 past 6. What is the angle between the hands? (1250)
Notes for OHTs:
8A.1 Alternative starter. What can you see?
Aim to generate and establish vocabulary. Focus on acute angles.
8A.2
Parallel lines with one transversal line.
A second OHT with a single straight line (different colour) can be used in conjunction with
this for the mini-plenary. What shapes can we make?
8A.3
What is this?  a clock. Without drawing the clock-face, draw hands to show 10 past 6.
Buckinghamshire KS3 Developing Geometrical Reasoning