Problem 27 - Elementary Algebra

Question

Construct a mathematical model given the following. \(y\) varies jointly as \(x\) and \(z,\) where \(y=2 / 3\) when \(x=1 / 2\) and \(z=12\).

Step-by-Step Solution

Answer

The mathematical model is \( y = \frac{1}{9}xz \).

Step 1: Understanding Joint Variation

In joint variation, a variable is directly proportional to the product of two or more other variables. In this case, the equation for joint variation can be represented as \( y = kxz \), where \( k \) is the constant of proportionality.

Step 2: Substituting Known Values

We are given that when \( y = \frac{2}{3} \), \( x = \frac{1}{2} \), and \( z = 12 \), these values satisfy the equation \( y = kxz \). Substituting these values into the equation gives \( \frac{2}{3} = k \times \frac{1}{2} \times 12 \).

Step 3: Solving for the Constant k

Rewrite the equation \( \frac{2}{3} = k \times \frac{1}{2} \times 12 \) to find \( k \). First, solve for \( \frac{1}{2} \times 12 = 6 \). This simplifies the original equation to \( \frac{2}{3} = k \times 6 \). Therefore, \( k = \frac{2}{3 \times 6} = \frac{1}{9} \).

Step 4: Formulating the Mathematical Model

Now that we've found the constant \( k = \frac{1}{9} \), substitute it back into the joint variation model \( y = kxz \). Therefore, the mathematical model is \( y = \frac{1}{9}xz \).

Key Concepts

Proportionality

Proportionality is a central concept in understanding how different variables relate to one another in mathematics. When we say a variable is proportional to another, we mean that the ratio of these variables remains constant. In joint variation, like the example in this exercise, a variable (here, \( y \)) is proportional to the product of two other variables (\( x \) and \( z \)). This means if we multiply or divide one of these variables by a constant, the variable on the other side of the equation will change to maintain that constant ratio.

The idea of proportionality is essential in understanding relationships in a wide range of subjects, from physics to economics. It helps us see how changes in one area might affect another, maintaining a balanced relationship across the board.

• When variables are proportional, increasing one variable increases the other, and vice versa.
• In our joint variation model, if \( x \) or \( z \) increases, \( y \) will also increase, provided the constant \( k \) stays the same.
• This forms a direct relationship that can be very useful for predictions and calculations.

Mathematical Model

A mathematical model is a formal way of representing relationships between variables using mathematical expressions. In this exercise, we created a model to describe how \( y \) varies with \( x \) and \( z \). By using the equation \( y = kxz \), we form a mathematical model that is precise and allows us to predict the behavior of one variable based on changes in others.

Creating a mathematical model involves identifying the kind of variation (joint, in this case), recognizing all variables involved, and determining the constant.

• Mathematical models like these help us simplify complex relationships into understandable, manageable terms.
• By substituting known values into the model, we can solve for unknowns, providing insights into real-world situations.
• The model \( y = \frac{1}{9}xz \) we discovered is a straightforward depiction of the interaction between \( y \), \( x \), and \( z \).

With these models, not only can we solve equations, but we can translate abstract mathematical ideas into practical, tangible insights.

Constant of Variation

In solving joint variation problems, finding the constant of variation \( k \) is crucial. The constant \( k \) serves as the fixed multiplier that relates the product of \( x \) and \( z \) to \( y \). In the exercise, we found \( k \) by rearranging our known equation to solve for \( k \): \( \frac{2}{3} = k \times \frac{1}{2} \times 12 \), which simplifies to \( k = \frac{1}{9} \).

The constant of variation is important because it implies a steady ratio between the variables involved, showing consistent proportionality.

• Once determined, \( k \) allows us to complete the mathematical model, offering clarity and consistency.
• \( k \) remains unchanged unless the relationship between the variables is altered.
• It gives insight into how dependent one variable is on others when all else remains unchanged.

Learning to calculate and apply \( k \) strengthens problem-solving skills and deepens understanding of variation and proportional relationships.