Congruence and similarity

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In order to access this I need to be confident with:

2D shapes Types of triangles Arithmetic Ratio Angle rules

This topic is relevant for:

GCSE Maths Geometry and Measure

Congruence And Similarity

Here we will learn about congruence and similarity, including what congruent shapes are and what similar shapes are. This will extend to congruent triangles.

There are also congruence and similarity worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

What is congruence and similarity?

Congruence and similarity are mathematical terms used to describe how two or more shapes are related to each other.

What is congruence and similarity?

Congruent shapes

Congruent shapes are exactly the same shape and the same size.

The corresponding sides are the same and the corresponding angles are the same. Shapes can be congruent if they have been reflected or rotated.

For example, these polygons are congruent.

Step-by-step guide: Congruent shapes

Congruent triangles

Congruent shapes can be extended to congruent triangles. There are $4$ special conditions to help us recognise congruent triangles. These conditions help us prove triangle congruence.

The $4$ reasons are

$SSS$ (side-side-side),

$RHS$ (right-angled triangle, hypotenuse and a side the same),

$ASA$ or $AAS$ (angle-side-angle or angle-angle-side),

$SAS$ (side-angle-side, two sides and the included angle).

Step-by-step guide: Congruent triangles

Similar shapes

Similar shapes are the same shape but they have different sizes. The corresponding sides are in the same ratio and the corresponding angles are the same.

If we know that $2$ shapes are mathematically similar we can work out any missing sides using the ratio, or scale factor.

For example, here are $2$ similar triangles.

The ratio of the sides is $4:12=1:3,$ the scale factor is $12 \div 4=3.$ We can use either of these to work out the size of the required length.

Therefore $x=3\times 7=21.$

Step-by-step guide: Similar shapes

How to use congruence and similarity

In order to answer congruence and similarity questions there are three key skills you need to know how to do. Each of which is explained in the examples below. These skills are,

Recognising congruent shapes
Recognising and proving congruent triangles
Recognising and using similar shapes

Explain how to use congruence and similarity

Congruence and similarity worksheet

Get your free congruence and similarity worksheet of 20+ questions and answers. Includes reasoning and applied questions.

COMING SOON

Congruence and similarity worksheet

Get your free congruence and similarity worksheet of 20+ questions and answers. Includes reasoning and applied questions.

COMING SOON

Congruence and similarity examples

Example 1: congruent shapes

Are these 2D shapes congruent?

Congruence and similarity example 1 image 1

Check the type of 2D shape.

Both shapes are rectangles.

2Check the corresponding angles and corresponding sides.

All the angles are $90^{\circ}.$

The short sides on both rectangles are $3.$

The long sides on both rectangles are $5.$

Congruence and similarity example 1 image 2

3State if the shapes are congruent or not.

The shapes are the same shape and the same size – they are congruent shapes.

Example 2: recognise congruent triangles

Decide whether this pair of triangles are congruent. If they are congruent, state why.

Congruence and similarity example 2 image 1

Check the corresponding angles and corresponding sides.

Both triangles have sides $9cm$ .

Both triangles have a $45^{\circ}$ angle.

But their second angles are different.

Congruence and similarity example 2 image 2

Decide if the shapes are congruent or not.

The triangles look like they are different shapes BUT the third angle can be worked out. Using the angle fact that the sum of interior angles of a triangle is $180^{\circ}$ we can work out the missing angle in both triangles.

Congruence and similarity example 2 image 3

The $9cm$ side is in between the angles $45^{\circ}$ and $35^{\circ}$ in both triangles. The triangles are the same shape and the same size. They are congruent triangles.

If the triangles are congruent, which congruence condition fits the pair of triangles.

The triangles are congruent with the condition angle-side-angle $(ASA).$

Example 3: prove congruent triangles (higher)

Prove that triangle $ABC$ is congruent to triangle $XYZ.$

Congruence and similarity example 3 image 1

Pair up the corresponding sides.

State which sides are identical, here there are two pairs of corresponding sides.

$\begin{aligned} AB &= XY \\\\ AC &= XZ \end{aligned}$

Congruence and similarity example 3 image 4

Pair up the corresponding angles.

State which angles are identical, here there is one pair of equal angles.

You need to use the correct notation.

$\text{angle} \ CAB = \text{angle} \ ZXY$

Congruence and similarity example 3 image 5

State which congruence condition fits the pair of triangles.

Triangle $ABC$ is congruent to triangle $XYZ$ because they fit the side-angle-side $(SAS)$ condition.

Example 4: finding a missing length using similar shapes

Here are two similar shapes. Find the length $PQ.$

Congruence and similarity example 4 image 1

Decide which sides are pairs of corresponding sides.

Pair up the sides that have measurements. Make sure you pair up the side mentioned in the question.

The sides $AB$ and $PQ$ are a pair of corresponding sides.

The sides $BC$ and $QR$ are a pair of corresponding sides.

Find the scale factor.

The ratio of the lengths $BC:QR$ is $8:6$ which simplifies to $4:3.$

This gives a scale factor of enlargement from triangle $ABC$ to triangle $PQR$ of $\frac{3}{4}.$

Use the scale factor to find the missing length.

The ratio of the lengths $AB:PQ$ is also $4:3.$

We can use the scale factor $\frac{3}{4}$ as a multiplier to find the missing length.

Congruence and similarity example 4 image 2

The missing side has been found, $PQ = 9cm.$

Alternatively an equation may be formed and solved.

Congruence and similarity example 4 image 3

Example 5: finding a missing area using similar shapes (higher)

These two figures are similar.

The area of shape $A$ is $10cm^{2}.$

Find the area of shape $B.$

Congruence and similarity example 5 image 1

Find the scale factor.

Use the given information to write a ratio and work out the scale factor.

When writing the ratios the order is very important.

$\begin{aligned} \text{The ratio of the lengths is } \quad \quad \text{length } A&: \text{length } B\\ 6&:12\\ \text{which simplifies to} \hspace{2cm} 1&:2 \end{aligned}$

The scale factor of enlargement from shape $A$ to shape $B$ is $2.$

Use the scale factor to find the missing value.

As we are finding an area we need to square the ratio of the lengths, and square the scale factor.

$\begin{aligned} \text{The ratio of the lengths is } \quad \quad \text{length} \ A&:\text{length } B\\ 1&:2\\ \text{the ratio of the areas is } \hspace{1cm} \text{area} \ A&:\text{area} \ B\\ 1^2&:2^2\\ \text{which simplifies to } \hspace{2cm} 1&:4 \end{aligned}$

We can use the area scale factor $2^2$ or $4$ as a multiplier to find the missing area.

Congruence and similarity example 5 image 2

$10\times 4=40$

The area of shape $B$ is $40cm^{2}.$

Alternatively you can form an equation to solve.

Congruence and similarity example 5 image 3

Example 6: finding a missing volume using similar shapes (higher)

These two shapes are similar.

The volume of shape $A$ is $120cm^{3}.$

Find the volume of shape $B.$

Congruence and similarity example 6 image 1

Find the scale factor.

Use the given information to write a ratio and work out the scale factor.

When writing the ratios the order is very important.

$\begin{aligned} \text{The ratio of the lengths is } \quad \quad \text{length } A&: \text{length } B\\ 5&:15\\ \text{which simplifies to} \hspace{2cm} 1&:3 \end{aligned}$

The scale factor of enlargement from shape $A$ to shape $B$ is $3.$

Use the scale factor to find the missing value.

As we are finding a volume we need to cube the ratio of the lengths, and cube the scale factor.

$\begin{aligned} \text{The ratio of the lengths is } \ \quad \quad \text{length} \ A&:\text{length} \ B\\ 1&:3\\ \text{The ratio of the volumes is } \hspace{0.7cm} \text{volume} \ A&:\text{volume} \ B\\ 1^3&:3^3\\ \text{which simplifies to } \hspace{2cm} 1&:27 \end{aligned}$

We can use the volume scale factor $3^3$ or $27$ as a multiplier to find the missing volume.

Congruence and similarity example 6 image 2

$120\times 27=3240$

The volume of shape $B$ is $3240cm^{2}.$

Alternatively you can form an equation to solve.

Congruence and similarity example 6 image 3

Common misconceptions

Shapes can be congruent but in different orientations

The second shape may be in a different orientation to the first shape. The shapes can still be congruent. Perhaps use tracing paper to help you check.

Shapes can be congruent but mirror images

The second shape may be a mirror image of the first shape. The shapes can still be congruent. Perhaps use tracing paper to help you check.

In most diagrams the diagrams are NOT drawn to scale

Questions about congruent shapes are often on grids. But sometimes diagrams may have shapes which are NOT drawn to scale. So use the measurements given, rather than measuring for yourself.

Scaling up or down

If you are finding a missing length in the larger shape you can multiply by the scale factor. The scale factor will be a number greater than $1.$

If you are finding a missing length in the smaller shape you can multiply by the scale factor, but the scale factor will be a number between $0$ and $1.$

Practice congruence and similarity questions

A

B

C

D

Shape $B$ is the same as the original shape, but is upside down.

ASA

SAS

SSS

RHS

The triangles are right-angled triangles. They have the same hypotenuse and the same short side.

Congruence and similarity practice question 2 image 2

SSS

Because

\begin{aligned} \\ AB &= PQ \\ AC &= RQ\\ BC &= PR \end{aligned}

SSS

Because

\begin{aligned} \\ AB &= RQ \\ AC &= PQ \\ BC &= PR \end{aligned}

SSS

Because

\begin{aligned} \\ AB &= PR \\ AC &= RQ\\ BC &= PQ \end{aligned}

SSS

Because

\begin{aligned} \\ AB &= PR \\ BC &= PQ\\ AC &= RQ \end{aligned}

The corresponding sides need to be paired up.

Congruence and similarity practice question 3 image 3

x=3.5

x=3

x=4

x=4.5

The ratio of the bases is $5:20$ which simplifies to $1:4.$

The scale factor of enlargement is $4.$ But we will need to divide by $4$ as we are finding a length on the smaller shape.

x=12\div4=3

240cm^2

120cm^2

180cm^2

200cm^2

The ratio of the bases is $10:20$ which simplifies to $1:2.$

The scale factor of enlargement is $2.$ But we are finding the area, so we need to multiply the area of shape $A$ by $2^2$ or $4.$

60\times 2^2=60\times 4=240

So the area of shape $B$ is $240cm^{2}.$

4480cm^3

560cm^3

8960cm^3

2240cm^3

The ratio of the bases is $10:40$ which simplifies to $1:4.$

The scale factor of enlargement is $4.$ But we are finding the volume, so we need to use $4^3$ or $64.$ Since we are finding the volume of the smaller shape we divide by the scale factor.

35840\div 4^3=35840\div 64=560

So the volume of shape $A$ is $560cm^{3}.$

Congruence and similarity GCSE questions

1. Which shapes are congruent to shape $X?$

Congruence and similarity gcse question 1

(2 marks)

Show answer

$A$ and $C$

(2)

2. Rectangles $ABCD$ and $PQRS$ are similar.

Congruence and similarity gcse question 2

\begin{aligned} &AC = 4 \ cm \\ &CD = 9 \ cm \\ &PQ = 12 \ cm \end{aligned}

Work out the length $QR.$

(2 marks)

Show answer

9 \times \frac{12}{4}

(1)

27cm

(1)

3. $ACD$ and $BCE$ are straight lines.

Congruence and similarity gcse question 3

$AB$ and $DE$ are parallel.

\begin{aligned} &AB = 16.3 \ cm \\ &AC = 11.2 \ cm \\ &CD = 5.6 \ cm \\ &CE = 4.8 \ cm \end{aligned}

(a) Calculate the length $BC.$

(b) Calculate the length $DE.$

(4 marks)

Show answer

(a)

4.8 \times \frac{11.2}{5.6}

(1)

9.6cm

(1)

(b)

16.3 \div 2

(1)

8.15cm

(1)

Learning checklist

You have now learned how to:

Recognise congruent shapes
Identify congruent shapes
Identify congruent triangles and their conditions of congruence
Prove two triangles are congruent (higher)
Find missing lengths of similar shape
Find areas and volumes of similar shapes (higher)

The next lessons are

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