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Stage 5.1-3

5.02 Similar triangle tests

Lesson

Introduction

Two triangles are considered to be similar if one of them can be scaled up or down in size, and then rotated and/or reflected to match the other.

We can think of similarity as a weaker version of congruency, where corresponding distances do not need to be equal but instead are in some fixed ratio.

The fixed ratio of distances between two similar figures can be referred to as the scale factor.

Similarity tests for triangles

There are four tests that we can use to test similarity between two triangles. If any one of these tests is satisfied then the two triangles must be similar. These four tests are:

  • AAA: Three pairs of equal angles

  • SSS: Three pairs of sides in the same ratio

  • SAS: Two pairs of sides in the same ratio and an equal included angle

  • RHS: Both have right angles, and the hypotenuses and another pair of sides are in the same ratio

These similarity tests can be proved to work in the same way that the congruency tests work, except with sides in fixed ratio rather than needing to be equal.

It is worth noting that, since all triangles have a fixed angle sum of 180^{ \circ{}}, having two matching angles is equivalent to having three matching angles.

To determine which sides are corresponding in two potentially similar triangles, we can use their positions with respect to any matching angles. If there is a common sized angle in both triangles, then the sides opposite those angles will be corresponding.

wo triangles with sides x and y opposite angles of the same size.

In this case, the sides labelled x and y are corresponding since they are opposite the same sized angle.

Similarly, if side lengths are given and two pairs of sides are in a fixed ratio between the two triangles, the angles between these pairs of sides will be corresponding.

Two triangles with an equal angle between the given sides. Ask your teacher for more information.

In this case, since the side pairs 5 & 8 and 10 & 16 are in the fixed ratio of 1:2, the angles between these pairs are corresponding and therefore equal.

Examples

Example 1

Consider the two triangles in the diagram below:

Triangles A B C and D E F where A B is 5, A C is 8, and angle A is 84 degrees. DE is 10, DF is 16 and angle D is 84 degrees.

Are \triangle ABC and \triangle DEF similar?

A
Yes, they satisfy AAA.
B
Yes, they satisfy SSS.
C
Yes, they satisfy SAS.
D
Yes, they satisfy RHS.
E
No, they are definitely not similar.
F
Unknown, there is not enough information.
Worked Solution
Create a strategy

Check the ratio of the given sides.

Apply the idea

Based on the given diagram, two triangles have equal angles of 84^{\circ{}}.

Sides AB and DE are corresponding, and sides AC and DF are corresponding.

\displaystyle \dfrac{DE}{AB}\displaystyle =\displaystyle \dfrac{10}{5}Find the ratio of the sides
\displaystyle =\displaystyle 2Evaluate
\displaystyle \dfrac{DF}{AC}\displaystyle =\displaystyle \dfrac{16}{8}Find the ratio of the sides
\displaystyle =\displaystyle 2Evaluate
\displaystyle =\displaystyle \dfrac{DE}{AB}Compare the ratios

Now we know two pairs of sides are in the same ratio, and the included angles are equal.

This means that the two triangles satisfy SAS. Option C is the correct answer.

Example 2

Consider the following diagram:

Two triangles D A E and B A C with a common angle of A. D E and B C are parallel. A E has length 6 and E C has length 10.
a

Are \triangle ABC and \triangle ADE similar?

Worked Solution
Create a strategy

Determine if they have any equal angles.

Apply the idea

\triangle ABC and \triangle ADE have a common angle at A.

DE and BC are parallel, so \angle ADE = \angle ABC since they are alternate angles on parallel lines.

Similarly, \angle AED = \angle ACB.

The two triangles are similar since they have three pairs of equal angles, AAA

b

Given that DE=7, what will the length of BC be?

Worked Solution
Create a strategy

Equate the ratios of corresponding sides.

Apply the idea
\displaystyle \dfrac{BC}{DE}\displaystyle =\displaystyle \dfrac{AC}{AE}Equate the ratios of the sides
\displaystyle \dfrac{BC}{7}\displaystyle =\displaystyle \dfrac{16}{6}Substitute the lengths
\displaystyle \dfrac{BC}{7} \times 7\displaystyle =\displaystyle \dfrac{16}{6} \times 7Multiply both sides of equation by 7
\displaystyle BC\displaystyle =\displaystyle \dfrac{56}{3} Evaluate the product
Idea summary

There are four tests that we can use to test similarity between two triangles:

  • AAA: Three pairs of equal angles

  • SSS: Three pairs of sides in the same ratio

  • SAS: Two pairs of sides in the same ratio and an equal included angle

  • RHS: Both have right angles, and the hypotenuses and another pair of sides are in the same ratio

Once we know that two triangles are similar, we can use corresponding angles and side lengths to find unknown angles or sides of either triangle.

Outcomes

MA5.1-11MG

describes and applies the properties of similar figures and scale drawings

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