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What is a Proportional Relationship? 

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What is a Proportional Relationship? 
CCSS.MATH.CONTENT.7.RP.A.2

Basics on the topic What is a Proportional Relationship? 

Proportional Relationships – Definition

In everyday life, we often encounter situations where understanding the relationship between different quantities is essential, such as when mixing ingredients for a recipe or determining the speed needed to travel a certain distance in a given time. This is where the concept of a proportional relationship becomes crucial, helping us make sense of these everyday scenarios logically and mathematically.

A proportional relationship is a relationship between two variables where their ratios or rates remain constant. This means that as one quantity increases or decreases, the other does so in a directly proportional manner.

Proportional relationships are all about maintaining a consistent ratio which can be represented in the following forms:

  • table
  • graph
  • equation

    Proportional Relationships in Tables

A proportional relationship requires a constant ratio between the two quantities. In this table, for each additional day, the money saved increases by $\$$5. This consistent increase indicates a fixed ratio.

ToV_27295_What_is_a_proportional_relationship-01.svg

This table shows that for each day, the amount of money saved increases by $\$$5, maintaining a consistent ratio of 1 day to $\$$5.

Understanding Direct Proportionality

Direct Proportionality: The table shows that the days and the amount of money saved are directly proportional. As the number of days increases, the total amount of money saved increases in a directly proportional manner.

Check how well you already understand direct proportionality with some exercises:

x y
1 3
2 6
3 9
Is the relationship shown in the table above proportional, and why?
x y
1 4
2 7
3 11
Is the relationship shown in the table above proportional, and why?

Identifying proportional relationships in tables helps you better understand and interpret data, a key skill in both math and everyday life.

Inverse Proportional Relationships

Inverse proportional relationships occur in many real-world scenarios, offering a different perspective on how quantities relate to each other compared to direct proportionality.

In an inverse proportional relationship, two variables behave in a way that when one increases, the other decreases proportionally, such that their product is always constant. This relationship is represented by the formula $y = \frac{k}{x}$, where $k$ is a constant.

Consider a scenario involving the brightness of a projector screen as you move further away.

  1. Closer Distance: When you are $2$ meters away from the screen, the brightness is at a certain level.
  2. Increasing Distance: As you move further away, say to $4$ meters, the brightness decreases.
  3. Relationship: If the brightness at $2$ meters is $B$, at 4 meters it would be $\frac{B}{2}$.

In this case, the brightness of the screen is inversely proportional to the distance from it. The further you move away, the less bright the screen appears. The product of the distance and brightness remains constant, illustrating an inverse proportional relationship.

Proportional Relationships in Graphs

Imagine a scenario where you save $5 every day. The graph of your savings over time would show a straight line, where the horizontal axis represents the days and the vertical axis represents the amount of money saved.

ToV_27295_What_is_a_proportional_relationship-02.svg

In a graph depicting a proportional relationship, the key feature is a straight line that intersects the origin. This line is a visual interpretation of the equation $y = kx$, where $k$ stands for the constant of proportionality. It demonstrates that every value of $y$ is directly determined by multiplying the corresponding $x$ value by the constant factor $k$, thus showcasing a precise proportional relationship between the two variables.

Let’s apply your knowledge of proportional relationships in graphs to practice now. ToV_27295_What_is_a_proportional_relationship-03.svg

Is this relationship proportional, and how do you know?

ToV_27295_What_is_a_proportional_relationship-04.svg

Is this relationship proportional, and why or why not?

Interpreting graphs of proportional relationships will also help you better understand the importance of the slope intercept form.

Proportional Relationships in Equations

An equation represents a proportional relationship if it can be expressed in the form $y = kx$, where $k$ is a constant.

ToV_27295_What_is_a_proportional_relationship-05.svg

The relationship between the days and the amount of money saved can be described by the equation $y = 5x$, where $y$ is the total amount of money saved, and $x$ is the number of days. This equation shows that the amount saved is directly proportional to the number of days.

Is the relationship described by the equation, $y= 5x$ proportional, and how do you know?
Is the relationship described by the equation, $y = 2x + 3$ proportional, and why or why not?

Representing proportional relationships with equations is a method that will directly lead to understanding what the slope of a line is.

Proportional Relationships – Real-World Problems

Let's work through some examples of real-world problems and determine whether there are proportional relationships.

Josh gets paid $\$$10 per hour for his part-time job. The relationship between his total earnings ($E$) and hours worked ($h$) can be represented by the equation $E = 10h$. Is this relationship proportional?
Hours (h) Distance (km)
1 60
2 120
3 180
Amy is traveling by train, which moves at a constant speed. Is the relationship between hours and distance traveled proportional? Refer to the table above.

ToV_27295_What_is_a_proportional_relationship-06.svg

Is the relationship between time and pages read proportional? Refer to the graph above.
Sam's phone plan charges $\$$0.05 per text message sent. The cost (C) for sending text messages (t) can be represented by the equation $C = 0.05t$. Is this relationship proportional?
Days (d) Water in Liters (L)
1 2
2 4
3 6
A plant needs a consistent amount of water daily. Does this table show a proportional relationship between days and water consumption? Refer to the table above.

ToV_27295_What_is_a_proportional_relationship-07.svg

Is the relationship between weeks and the amount saved proportional? Refer to the graph above.

Proportional Relationships – Summary

Key Learnings from this Text:

  • A proportional relationship maintains a constant ratio between two quantities.
  • In tables, this is seen as a constant ratio across rows.
  • In graphs, proportional relationships are represented by straight lines through the origin.
  • In equations, they follow the form $y = kx$, where $k$ is a constant.

Proportional Relationships – Frequently Asked Questions

What is a proportional relationship?
How can you identify a proportional relationship in a table?
What does a proportional relationship look like on a graph?
Can you give an example of a proportional relationship equation?
Why is understanding proportional relationships important?
How is a proportional relationship different from an inverse relationship?
Can a proportional relationship have a negative constant of proportionality?
How do you find the constant of proportionality in a graph?
Is every linear relationship a proportional relationship?
Can two quantities be proportional if one of them is always zero?
What is the difference between a ratio and a proportion?
How is direct variation related to proportional relationships?
Can proportions be used to solve real-life problems?
How do proportional relationships apply in science?

Transcript What is a Proportional Relationship? 

[Embark on an extraordinary journey as the world's most perfect smoothie is unveiled, captivating all who taste its flawless perfection! Sadly, the serving size is only for one. To discover the secret to recreating this smoothie for the world to enjoy, we will need to ask the question, What is a Proportional Relationship? A proportional relationship exists when two variables vary directly with each other. When we analyze the relationship between variables in a table, we determine if the ratios are equivalent. If they are all equivalent, then the relationship is proportional. If you notice in the table, the ratio from x to y is one-fifth in the top row. The second row is two-tenths, which is equivalent to one-fifth. As you can see, all of the remaining ratios are also equal to one-fifth. If we looked at it in reverse and compared the y to x ratio, the ratios will still be equivalent! All of the ratios are equivalent to five, so this relationship is proportional. If we had this information as a graph, there are two features we need to look at to determine if it's proportional. The line must pass through the origin, and it must be a perfectly straight line to be a proportional relationship. A non-proportional relationship in table form, would yield ratios that are not all equivalent to each other. Whether we compare the x to y as a ratio, or the reverse y to x, this is a non-proportional relationship. Take a look at what a non-proportional relationship looks like when graphed. Our line does not go through the origin and it is not straight. Even if one of these conditions isn't true, then the relationship is non-proportional. Okay, now that we have these tools, let's get back to those smoothies. Let's look closely at the ratios in the table, to examine the relationship between the number of servings, and cups of yogurt. For one serving it would be two cups of yogurt per serving. For two servings, it would also be two cups of yogurt per serving. But it is different at five servings! The ratio is not equivalent because it would be three cups of yogurt per serving, which will definitely impact the taste. This table is non-proportional because the ratios are not all equivalent. For the next smoothie ingredient, let's look at the relationship between servings and ice cubes used and determine if it's proportional. The line is straight, which is one of the two conditions that a proportional relationship must meet. However, because this line does not pass through the origin, this relationship is non-proportional. The next smoothie ingredient is bananas. Each of these ratios is equivalent to one, so this is in fact a proportional relationship. The last ingredient in the smoothie is drops of spicy habanero sauce. Let's examine the relationship shown. This graph shows a proportional relationship since the line passes through the origin, and it is perfectly straight. Now that we know about those smoothie ingredients, let's summarize what we learned! A relationship is considered proportional if all its ratios are equivalent, and it can create a straight line that passes through the origin. Unlock the realm of perfection with our meticulously equivalent ingredients, crafting the world's most proportional smoothie.