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Lesson 6 of 10
  • Year 8

Diagonals of a rhombus

I can appreciate that the diagonals of a rhombus bisect one another at right angles and the angles are bisected.

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Lesson 6 of 10
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  • Year 8

Diagonals of a rhombus

I can appreciate that the diagonals of a rhombus bisect one another at right angles and the angles are bisected.

Download all resources
Share activities with pupils

These resources will be removed by end of Summer Term 2025.

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These resources were created for remote use during the pandemic and are not designed for classroom teaching.

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Lesson details

Key learning points

  1. An isosceles triangle has a line of symmetry that splits it into two congruent triangles.
  2. In a rhombus each diagonal splits the rhombus into two congruent isosceles triangles bisecting the angles.
  3. The two diagonals meet at right angles.
  4. The two diagonals bisect each other.

Keywords

  • Bisect - To bisect means to cut or divide an object into two equal parts.

  • Altitude - The altitude of a triangle is a line segment from a vertex of a triangle to the opposite edge. It is perpendicular to that edge.

Common misconception

Pupils may struggle with a diagonal of a shape being vertical or horizontal.

By reminding pupils of the definition, ask them to check if it fits and the shape could be rotated as well.


To help you plan your year 8 maths lesson on: Diagonals of a rhombus, download all teaching resources for free and adapt to suit your pupils' needs...

You can set up pair or group work to explore the question 'Is a square a rhombus?' Pupils will need to use good mathematical vocabulary and reasoning to explain their argument. Then you could have them consider 'is a rhombus a square?'.
Teacher tip

Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Prior knowledge starter quiz

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6 Questions

Q1.
If two lines are perpendicular to each other then they create a $$^\circ$$ angle.

Correct Answer: 90 , ninety

Q2.
Match the type of triangle to the interior angles.

Correct Answer:Scalene triangle,74°, 36° and 70°

74°, 36° and 70°

Correct Answer:Isosceles triangle,28°, 124° and 28°

28°, 124° and 28°

Correct Answer:Right-angled triangle,30°, 60° and 90°

30°, 60° and 90°

Correct Answer:Equilateral triangle,60°, 60° and 60°

60°, 60° and 60°

Q3.
How many lines of symmetry does a rhombus have?

0
1
Correct answer: 2
3
4

Q4.
The missing angle in this triangle is °.

An image in a quiz
Correct Answer: 47, forty seven

Q5.
Select all of the properties of a rhombus.

It has three equal edges.
Correct answer: It has equal edges.
Correct answer: It has four interior angles.
It has one set of parallel sides.
Correct answer: Opposite angles are equal.

Q6.
Which of these is not a construction of a rhombus?

Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz

Assessment exit quiz

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6 Questions

Q1.
To means to cut or divide an object into two equal parts.

Correct Answer: bisect

Q2.
How many of the altitudes of an isosceles are also a line of symmetry?

0
Correct answer: 1
2
3

Q3.
The diagonals of a rhombus ...

Correct answer: ...are perpendicular to each other.
Correct answer: ...bisect each other.
...are always equal in length.
Correct answer: ...bisect the angle they pass through.
...cannot be vertical or horizontal.

Q4.
In this diagram of a rhombus and its diagonals, the angle marked b is °.

An image in a quiz
Correct Answer: 116, one hundred and sixteen

Q5.
Which of the following statements are true about this image?

An image in a quiz
Correct answer: $$a=58+c$$
$$b = a + c$$
$$b=2a$$
Correct answer: $$\frac{b}{2}=58$$
$$b+c=180$$

Q6.
A rhombus has diagonals with lengths of 8 cm and 12 cm. Which of the following calculations will work out the area of the rhombus?

$$2\times\left(\frac{1}{2}\times8\times12\right)$$
Correct answer: $$2\times\left(\frac{1}{2}\times4\times12\right)$$
$$2\times\left(\frac{1}{2}\times4\times6\right)$$
Correct answer: $$4\times\left(\frac{1}{2}\times4\times6\right)$$
Correct answer: $$2\times\left(\frac{1}{2}\times8\times6\right)$$