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# Proportionality Theorems

What's a birthday party without cake! Most people are always excited to get to the end of the party because they get to eat cake. But not everybody knows that to bake a sweet cake, you need the right proportions of different ingredients. Your recipe may require 2 pounds of flour for one cake. That means if you want to bake two cakes, you'll need 4 pounds of flour. In maths, you can express this in form of a ratio and say the ratio of flour to one cake is and if you were to bake two cakes, it will be . These ratios are proportional to each other because they are equal.

## What are Proportionality Theorems in Geometry?

Proportionality theorems show relationships between shapes in the form of ratios. They show how different ratios of a figure or a quantity are equal. The proportionality theorems are mostly used in triangles. Let's look at the fundamental concept of the proportionality theorem using the triangle figures below.

Similar Triangles - StudySmarter Originals

The triangles above will be called similar triangles if their angles are congruent and if their corresponding sides are proportional. So, the proportionality formula for similar triangles is below.

## What is the Basic Proportionality Theorem?

The Basic Proportionality Theorem focuses on showing the relationship between the length of the sides of a triangle.

The proportionality theorem states that if a line is drawn parallel to one side of a triangle to intersect the other two sides at distinct points, then the other two sides are divided in the same ratio.

The figure below gives a visual representation of the theorem.

A Triangle - StudySmarter Originals

In the above, is parallel to . According to the Basic Proportionality Theorem, the ratio of to is equal to the ratio of to :

The ratio above is considered the basic proportionality formula.

We can prove this theorem and find out how to get the formula. Let's see how.

From the theorem, we know that and are in the same ratio and we want to prove that they are equal. We will first form triangles that have and as their side lengths. To get these triangles, we will draw a segment joining to and another segment joining to as shown below.

A triangle divided into parts with segments - StudySmarter Originals

We have now formed two new triangles.

The next thing is to find a relationship between the new triangles. In particular, let's look at the area. have the same base and the same height because the third vertex of the triangle is between the same parallel. Therefore, the area of both triangles must be equal:

Now, consider . Let's take as the base and the height as the perpendicular distance from the line to the opposite vertex . See how it looks like in the figure below.

A triangle divided into parts with segments - StudySmarter Originals

The area of this triangle is

We also need the area of which will be:

Now, we can take the ratio of the area of to the area of and compare it with the ratio of the area of to the area of . Therefore, the ratio of the areas is:

As you can see, we've got the first part of the formula. To get the other, we will repeat everything we just did but with.

Unlike before, instead of using as the base of , we will use as the base and the height will be the perpendicular distance opposite the vertex . See how it looks like in the figure below.

A triangle divided into parts with segments - StudySmarter Originals

The area of according to the image above is

Now let's consider the area of . We will take EC as the base and as the height. The area is as follows.

We will now get the ratio of both areas to be:

So, you can see that we've gotten the other part of the formula. But how do we show that both parts are equal? Let's equate both ratios and see.

Both numerators are the same so they are equal. Recall that at the beginning of the proof, we saw that

Therefore,

### The Triangle Proportionality Theorem and Fundamental Theorem of Proportionality

The Triangle Proportionality Theorem of Fundamental Theorem of Proportionality are just other names for the Basic Proportionality Theorem. You may see this theorem referred to as any of these titles!

## Proportionality Theorem Examples

Let's see the application of the proportionality theorem with some examples.

Consider a where DE is parallel to . . Find .

Remember the formula

All we have to do is substitute the values.

Let's take a look at another example.

Consider where and are parallel to each other. . Find

According to the proportionality theorem,

Subbing in the known values leaves us with

Aside from showing the relationship between the length of sides of triangles, in real life, the proportionality theorem can be used in construction.

## The converse of the Basic Proportionality Theorem

The converse of the basic proportionality theorem is the reverse of the basic proportionality theorem. The theorem states that if a line is drawn to intersect two sides of a triangle at different points such that it cuts the two sides in the same ratio, then the line is parallel to the third side.

A Triangle - StudySmarter Originals

In the basic proportionality theorem, we saw that and are parallel and now we want to prove that and are indeed parallel. We would do this using the basic proportionality theorem which is

This proof is proof by contradiction meaning that we will assume that our desired result is wrong. We will assume that is not parallel to (. If this is the case, then there must be another point on line such that a segment drawn from point to that point is parallel to . See the figure below for clarity.

A triangle divided into parts with segments - StudySmarter Originals

Now that we have a line segment that is parallel to , we can now use the basic proportionality theorem which is

If you consider the basic proportionality theorem, you will have:

We have now derived that is parallel to and we want to show that is parallel to . This means that what we really want to do is show that and are the same segments. So, if we consider the above equation, you will see that the first ratio is not really needed. So we are left with

We are now saying that and are the same segments which means that point and point are the same. If this is our conclusion, then the segment and are the same but we haven't really proven that yet.

From the figure, we can say that the segment is equal to the sum of the segment and .

Let's go back to one of our equations.

We will now add 1 (one) to both sides of the equation and bring them into the fractions by giving them a common denominator.

Both numerators on both sides of the equation are representations of the segment . So, we can replace them with

Let's simplify further by multiplying both sides by .

Since they are equal, their reciprocals will also be equal. Therefore,

You should observe that and are on the same line. If they are on the same line, the only way they can be equal is if both segments start at the same point. This means that point F must be equal to point . It also means that the segment is the same as .

This concludes that is indeed parallel to .

## Proportionality Theorems - Key takeaways

• The basic proportionality theorem states that if a line is drawn parallel to one side of a triangle to intersect the other two sides at distinct points, then the other two sides are divided in the same ratio. The figure below gives a visual representation of the theorem.
• The basic proportionality theorem is also referred to as the triangle proportionality theorem and proportionality segment theorem.
• The converse of the basic proportionality theorem is the reverse of the basic proportionality theorem. The theorem states that if a line is drawn to intersect two sides of a triangle at different points such that it cuts the two sides in the same ratio, then the line is parallel to the third side.

Proportionality theorems are for the purpose of showing relationships in form of ratios. They show how different ratios of a figure or a quantity are equal.

You don't solve the theorem, you use the theorem to solve problems.

The basic proportionality theorem formula when there is a triangle ABC with a parallel segment DE is AD/DB = AE/EC

The proportionality segment theorem is the same as the basic proportionality theorem.

The proportionality theorem is used to show the relationship between the length of sides of triangles. In real life, it can be used in construction.

## Final Proportionality Theorems Quiz

Question

What is the basic proportionality theorem?

The basic proportionality theorem states that if a line is drawn parallel to one side of a triangle to intersect the other two sides at distinct points, then the other two sides are divided in the same ratio.

Show question

Question

The basic proportionality theorem is also called the triangle proportionality theorem.

True

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Question

What is the converse of the basic proportionality theorem?

The converse of the basic proportionality theorem states that if a line is drawn to intersect two sides of a triangle at different points such that it cuts the two sides in the same ratio, then the line is parallel to the third side.

Show question

Question

The converse proportionality theorem is the reverse of the basic proportionality theorem.

True

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Question

What can proportionality theorems be used for?

Proportionality theorems are for the purpose of showing relationships in form of ratios.They are used to find the relationship between lengths of sides of figures and it can also be used in construction.

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Question

How do you solve proportionality theorem?

You don't solve the theorem, you use the theorem to solve problems!

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Question

What is the Fundamental Theorem of Proportionality?

The Fundamental Theorem of Proportionality is the same as the basic proportionality theorem.

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Question

The proportionality theorem can be used in construction. True or False

True

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Question

The proportionality theorem is used to show the relationship between the length of sides of triangles.

True

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Question

What are the other names for proportionality theorem?

The triangle proportionality theorem and the fundamental theorem of proportionality.

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