Calculating the Percentage Change in q

Oct 13, 2025 by wbrum.org 39 views

Understanding Direct Variation

Direct variation is a relationship between two variables where one variable is a constant multiple of the other. In mathematical terms, if $q$ varies directly as the square of $R$ , we can write the equation as:

q = kR^2

where $k$ is a constant of proportionality.

Applying the Concept to a Real-World Scenario

Let's say we have a situation where the value of $q$ is directly proportional to the square of $R$ . For example, the volume of a sphere is directly proportional to the square of its radius. If the radius of the sphere decreases by 20%, we want to find the percentage change in the volume.

Calculating the Percentage Change in q

To find the percentage change in $q$ , we need to first find the new value of $q$ when $R$ decreases by 20%. Let's assume the original value of $R$ is $R_0$ and the original value of $q$ is $q_0$ . When $R$ decreases by 20%, the new value of $R$ is:

R_{new} = R_0 - 0.2R_0 = 0.8R_0

Now, we can find the new value of $q$ using the equation:

q_{new} = k(0.8R_0)^2 = 0.64kR_0^2

Since $q_0 = kR_0^2$ , we can write:

q_{new} = 0.64q_0

Finding the Percentage Change

To find the percentage change in $q$ , we can use the formula:

\text{Percentage Change} = \frac{q_{new} - q_0}{q_0} \times 100\%

Substituting the values, we get:

\text{Percentage Change} = \frac{0.64q_0 - q_0}{q_0} \times 100\% = -36\%

Conclusion

In this article, we explored the concept of direct variation and its application to a real-world scenario. We found that if $q$ varies directly as the square of $R$ and $R$ decreases by 20%, the percentage change in $q$ is -36%.

Key Takeaways

Direct variation is a relationship between two variables where one variable is a constant multiple of the other.
If $q$ varies directly as the square of $R$ , we can write the equation as $q = kR^2$ .
When $R$ decreases by 20%, the new value of $q$ is 0.64 times the original value of $q$ .
The percentage change in $q$ is -36%.

References

[1] "Direct Variation" by Khan Academy
[2] "Direct Variation" by Math Is Fun
[3] "Direct Variation" by Wolfram MathWorld
Direct Variation Q&A =========================

Frequently Asked Questions

In this article, we'll answer some of the most frequently asked questions about direct variation.

Q: What is direct variation?

A: Direct variation is a relationship between two variables where one variable is a constant multiple of the other. In mathematical terms, if $q$ varies directly as the square of $R$ , we can write the equation as:

q = kR^2

where $k$ is a constant of proportionality.

Q: What is the difference between direct variation and inverse variation?

A: Direct variation is a relationship where one variable is a constant multiple of the other, while inverse variation is a relationship where one variable is a constant divided by the other. For example, if $q$ varies inversely as $R$ , we can write the equation as:

q = \frac{k}{R}

Q: How do I determine if a relationship is a direct variation?

A: To determine if a relationship is a direct variation, you can use the following steps:

Plot the data on a graph.
Check if the graph is a straight line.
If the graph is a straight line, the relationship is a direct variation.

Q: What is the constant of proportionality?

A: The constant of proportionality is a value that represents the ratio of the two variables in a direct variation. In the equation $q = kR^2$ , $k$ is the constant of proportionality.

Q: How do I find the constant of proportionality?

A: To find the constant of proportionality, you can use the following steps:

Choose two values of $R$ and the corresponding values of $q$ .
Plug the values into the equation $q = kR^2$ .
Solve for $k$ .

Q: What is the relationship between direct variation and percentage change?

A: If $q$ varies directly as the square of $R$ and $R$ decreases by 20%, the percentage change in $q$ is -36%. This is because the new value of $q$ is 0.64 times the original value of $q$ .

Q: Can direct variation be used to model real-world phenomena?

A: Yes, direct variation can be used to model real-world phenomena such as the volume of a sphere, the area of a circle, and the force of gravity.

Q: What are some common applications of direct variation?

A: Some common applications of direct variation include:

Physics: to model the motion of objects under the influence of gravity or friction.
Engineering: to design systems that require a constant ratio of variables.
Economics: to model the relationship between prices and quantities of goods.

Q: How do I use direct variation to solve problems?

A: To use direct variation to solve problems, you can follow these steps:

Identify the variables and the relationship between them.
Write the equation of direct variation.
Plug in the values and solve for the unknown variable.

Conclusion

In this article, we've answered some of the most frequently asked questions about direct variation. We've covered topics such as the definition of direct variation, the difference between direct and inverse variation, and the relationship between direct variation and percentage change. We've also discussed some common applications of direct variation and provided tips on how to use it to solve problems.

Key Takeaways

Direct variation is a relationship between two variables where one variable is a constant multiple of the other.
The constant of proportionality is a value that represents the ratio of the two variables in a direct variation.
Direct variation can be used to model real-world phenomena such as the volume of a sphere and the force of gravity.
The percentage change in $q$ is -36% if $R$ decreases by 20%.

References

[1] "Direct Variation" by Khan Academy
[2] "Direct Variation" by Math Is Fun
[3] "Direct Variation" by Wolfram MathWorld