# How to Use Graphs to Write Proportional Relationship

Proportional relationships are a key mathematical concept used to describe a relationship between two variables where one is a constant multiple of the other. For instance, if $$y$$ is always twice as much as $$x$$, that's a proportional relationship. A graph is a great tool to illustrate these relationships visually. ## A Step-by-step Guide to Using Graphs to Write Proportional Relationship

Here are the step-by-step instructions to do this:

### Step 1: Identify the Variables

The first step is to identify the two variables you’ll be working with. Let’s say, for instance, you’re comparing the cost of apples $$(y)$$ to the number of apples you buy $$(x)$$.

### Step 2: Graph the Data

You’ll plot this data on a graph. In this case, the x-axis might represent the number of apples, and the y-axis might represent the cost. Each point on the graph will represent a specific scenario (e.g., $$3$$ apples cost $$6)$$.

### Step 3: Draw a Line of Best Fit

Next, draw a line of best fit through your data points. For a perfect proportional relationship, all points should lie on this line. However, in the real world, it’s often the case that data are merely “approximately” proportional, so don’t be surprised if your data don’t fit perfectly.

### Step 4: Identify the Origin Point

Check whether the line passes through the origin $$(0,0)$$. If it does, this means that when you have zero of one variable (no apples), you have zero of the other variable (no cost), confirming a proportional relationship.

### Step 5: Calculate the Slope

The slope of the line represents the rate of change or the ratio between the two variables. It can be calculated by choosing two points on the line and using the formula $$\frac{(change\:in\:y)}{(change\:in\:x)}$$. In our example, the slope would be the cost per apple.

### Step 6: Write the Equation

Once you have the slope, you can write the equation of the line. A proportional relationship can be written in the form $$y = mx$$, where $$m$$ is the slope. In our example, if the slope was $$2$$ (meaning each apple costs $$2)$$, our equation would be $$y = 2x$$.

### Step 7: Interpret the Equation

Finally, interpret the equation in the context of the problem. In our example, $$y = 2x$$ means that the total cost $$(y)$$ is $$2$$ times the number of apples $$(x)$$.

Remember that a proportional relationship means that the ratio between the two variables is always the same. So in our example, no matter how many apples we buy, the cost per apple remains constant at $$2$$. This relationship would be represented on the graph as a straight line passing through the origin with a slope of $$2$$.

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