# Linear graphs

Linear graphs depict relationships between two variables using straight lines, illustrating a constant rate of change. The general equation for a linear graph is y = mx + c, where 'm' is the gradient and 'c' represents the y-intercept. These graphs are fundamental in understanding patterns and making predictions in various fields such as mathematics, science, and economics.

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## Definitions of Linear Graphs

Linear graphs are an essential concept in mathematics, particularly in algebra and coordinate geometry. Before diving into its examples and applications, it's crucial to understand the key definitions associated with linear graphs.

### Definition of a Linear Graph

A linear graph is a graph that represents a linear equation. The most basic form of a linear equation is $$y = mx + c$$, where m is the slope and c is the y-intercept. In a linear graph, the relation between two variables can be represented as a straight line.

The slope (or gradient) of a linear graph depicts its steepness and direction. The slope is the ratio of the change in the y-coordinate to the change in the x-coordinate, often written as $$m = \frac{\Delta y}{\Delta x}$$. A positive slope means the line ascends from the left to the right, whereas a negative slope means it descends.

### Y-Intercept

The y-intercept is the point where the linear graph intersects the y-axis. It occurs when the value of x is zero. In the equation $$y = mx + c$$, the y-intercept is represented by c.

### Equation of a Line

The equation of a line in its most common form is $$y = mx + c$$. However, it can also be written in the standard form $$Ax + By = C$$ where $$A$$, $$B$$, and $$C$$ are constants. This form is especially useful when dealing with vertical lines where the slope is undefined.

• Consider the linear equation $$y = 2x + 3$$. Here, the slope is 2 and the y-intercept is 3.
• To find the coordinates for plotting, choose values for x and compute the corresponding y values. For instance:
• When x = 0, y = 3$$(0, 3)$$
• When x = 1, y = 5$$(1, 5)$$
• When x = 2, y = 7$$(2, 7)$$

### Horizontal and Vertical Lines

Horizontal and vertical lines are special cases of linear graphs.

• A horizontal line is represented by the equation $$y = c$$, where $$c$$ is a constant. Its slope is zero, meaning there is no change in the y-coordinate regardless of the x-coordinate.
• A vertical line is represented by the equation $$x = k$$, where $$k$$ is a constant. For vertical lines, the slope is undefined as the change in the x-coordinate is zero.

Remember, the slope of a vertical line cannot be determined because dividing by zero is undefined.

## How to Graph Linear Equations

Graphing linear equations is a crucial skill in mathematics. It allows you to visually interpret and analyse the relationship between two variables represented in a linear equation.

### Plotting Points to Graph

To graph a linear equation, you start by finding points that lie on the line. Follow these steps to plot points effectively:

• Choose values for x: Select a few values for x, such as -2, -1, 0, 1, and 2.
• Calculate corresponding y-values: Substitute each chosen x-value into the linear equation to find the corresponding y-value.
• Plot the points: Plot the (x, y) pairs on a coordinate plane.
• Draw the line: Connect the plotted points with a straight line.

Example: Consider the linear equation $$y = 3x - 2$$.

• When $$x = -2$$, $$y = 3(-2) - 2 = -6 - 2 = -8$$. This gives the point (-2, -8).
• When $$x = -1$$, $$y = 3(-1) - 2 = -3 - 2 = -5$$. This gives the point (-1, -5).
• When $$x = 0$$, $$y = 3(0) - 2 = -2$$. This gives the point (0, -2).
• When $$x = 1$$, $$y = 3(1) - 2 = 3 - 2 = 1$$. This gives the point (1, 1).
• When $$x = 2$$, $$y = 3(2) - 2 = 6 - 2 = 4$$. This gives the point (2, 4).

### Using the Slope-Intercept Form

The slope-intercept form of a linear equation is $$y = mx + c$$, where:

• $$m$$ is the slope of the line
• $$c$$ is the y-intercept, the point at which the line crosses the y-axis

The slope $$m$$ indicates the direction and steepness of the line. A positive slope means the line rises from left to right, while a negative slope means it falls from left to right. The y-intercept $$c$$ is the value of y when x is 0.

A quick way to plot a line is to start at the y-intercept and use the slope to find other points on the line.

### Graphing Using Intercepts

Another effective method to graph a linear equation is by using its intercepts. Intercepts are where the line crosses the axes.

• X-intercept: The point where the line crosses the x-axis (y = 0)
• Y-intercept: The point where the line crosses the y-axis (x = 0)

For the equation $$2x + 3y = 6$$:

• X-intercept: Set y to 0, then solve for x.$$2x + 3(0) = 6$$ =>$$2x = 6$$ =>$$x = 3$$. The x-intercept is (3, 0).
• Y-intercept: Set x to 0, then solve for y.$$2(0) + 3y = 6$$ => $$3y = 6$$ => $$y = 2$$. The y-intercept is (0, 2).

Even though linear equations typically result in straight lines, these lines can represent complex relationships in real-world situations. For instance, the salary you earn could be proportional to the number of hours you work, ignoring taxes and other deductions. Here, the linear equation would provide a simple but powerful model for understanding more about earnings. Understanding the intercepts and slopes can give insights into these situations, such as base pay (y-intercept) and hourly rate (slope).

## Techniques for Graphing Linear Equations

Understanding the various techniques for graphing linear equations can greatly enhance your ability to analyse and interpret data. Here we will explore multiple methods to graph linear equations effectively.

### Plotting Points

A common method for graphing linear equations is by plotting points. Follow these steps:

• Choose values for x: Select a range of x-values, such as -2, -1, 0, 1, and 2.
• Calculate corresponding y-values: Substitute the chosen x-values into the equation to find the corresponding y-values.
• Plot the points: Plot these (x, y) pairs on a graph.
• Connect the points: Draw a straight line through the plotted points.
• Consider the equation $$y = 2x + 1$$.
• When $$x = -2$$, $$y = 2(-2) + 1 = -3$$.
• When $$x = -1$$, $$y = 2(-1) + 1 = -1$$.
• When $$x = 0$$, $$y = 2(0) + 1 = 1$$.
• When $$x = 1$$, $$y = 2(1) + 1 = 3$$.
• When $$x = 2$$, $$y = 2(2) + 1 = 5$$.
This will yield the points (-2, -3), (-1, -1), (0, 1), (1, 3), and (2, 5).

### Using the Slope-Intercept Form

The slope-intercept form offers a quick way to graph a line using the equation $$y = mx + c$$, where m is the slope and c is the y-intercept.

The slope (m) represents the rate of change or the steepness of the line, given by $$m = \frac{\Delta y}{\Delta x}$$. The y-intercept (c) is the point where the line crosses the y-axis.

Starting from the y-intercept and then using the slope can help you quickly draw a graph without calculating multiple points.

### Graphing Using Intercepts

You can graph a linear equation by finding the x and y intercepts. Follow these steps:

• Find the y-intercept: Set x to 0 and solve for y.
• Find the x-intercept: Set y to 0 and solve for x.
• Plot these intercepts: Plot the points calculated in the previous steps.
• Draw a line: Connect the intercepts with a straight line.

For the equation $$3x + 2y = 6$$:

• Y-intercept: Set x to 0, giving $$2y = 6$$ or $$y = 3$$. The y-intercept is (0, 3).
• X-intercept: Set y to 0, giving $$3x = 6$$ or $$x = 2$$. The x-intercept is (2, 0).
Now plot these points and draw a line through them.

Even though linear equations typically result in straight lines, these lines can represent complex relationships in real-world situations. For example, the relationship between hours worked and earnings can be modelled with a linear equation if your earnings are directly proportional to hours worked, ignoring taxes and other deductions. The slope could represent your hourly wage and the y-intercept might represent any fixed earnings. Understanding these graphing techniques will enable you to analyse such relationships effectively.

## Examples of Linear Graphs

Linear graphs represent linear relationships between two variables. Here, we'll explore various examples to understand how linear equations translate into graphs.

### Solving Linear Graph Problems

Solving problems involving linear graphs often requires plotting points on a coordinate plane and drawing lines to represent linear equations. The steps include choosing values for x, calculating corresponding y values, and then plotting these points.

• Given the equation $$y = 2x + 4$$, let's solve it for two points.
• When $$x = -1$$, $$y = 2(-1) + 4 = 2$$. This gives the point (-1, 2).
• When $$x = 2$$, $$y = 2(2) + 4 = 8$$. This gives the point (2, 8).

### Understanding the Basics of Linear Graphs

Linear graphs are visual representations of linear equations. They help us understand the relationships between variables, slopes, and intercepts.

A linear equation is any equation that can be written in the form $$y = mx + c$$, where m and c are constants.

Understanding the underlying principles of linear graphs can make it easier to grasp more advanced concepts in mathematics. For example, the slope (m) explains how the lines behave with respect to changes in x and y. Recognising this relationship is crucial in calculus and physics.

### Step-by-Step Guide to Graphing Linear Equations

To graph a linear equation, follow these steps:

• Write the equation in slope-intercept form: Ensure your equation is in the form $$y = mx + c$$.
• Identify the slope and y-intercept: Here, m is the slope and c is the y-intercept.
• Plot the y-intercept: Locate the y-intercept on the y-axis.
• Use the slope to find another point: From the y-intercept, move according to the slope to find another point on the line.
• Draw the line: Connect the points with a straight line.

### Common Mistakes When Graphing Linear Equations

Avoid these common errors when graphing linear equations:

• Incorrectly calculating points: Ensure you substitute x-values accurately into the equation to get correct y-values.
• Misinterpreting the slope: Remember that the slope is $$m = \frac{\Delta y}{\Delta x}$$, where $$\Delta$$ indicates change.
• Forgetting to plot the y-intercept: Always start plotting from the y-intercept.

### Practical Applications of Linear Graphs in Real Life

Linear graphs are used in various everyday contexts such as economics, physics, and engineering. Here are some practical applications:

• Economics: Linear graphs can represent supply and demand curves, helping businesses predict profits.
• Physics: They can depict relationships between variables such as distance and time.
• Engineering: Linear graphs are used to model forces and trajectories.

### Practice Problems for Linear Graphs

Solve these problems to practise graphing linear equations:

• Graph the equation $$y = -3x + 2$$. Calculate at least three points.
• Solve for x and y intercepts of the equation $$5x - 4y = 20$$.

Double-check your calculations to ensure accuracy in graphing.

### Key Terms and Concepts for Linear Graphs

Understanding key terms and concepts is crucial for mastering linear graphs:

 Term Definition Linear Equation An equation of the form $$y = mx + c$$ Slope (m) The ratio of the change in y to the change in x Y-Intercept (c) The value of y when x = 0 X-Intercept The value of x when y = 0 Coordinate Plane A plane with a horizontal axis (x) and a vertical axis (y)

## Linear graphs - Key takeaways

• Linear graphs visually represent linear equations as straight lines, with the primary equation form being y = mx + c, where m is the slope and c is the y-intercept.
• The slope (or gradient) of a linear graph indicates its steepness and direction, calculated as the ratio of the change in the y-coordinate to the change in the x-coordinate, represented as m = Δy/Δx.
• The y-intercept is the point where the linear graph intersects the y-axis, taking the value c in the equation y = mx + c, while the x-intercept is found where the line intersects the x-axis.
• Graphing linear equations involves plotting points calculated by substituting chosen x-values into the equation to find corresponding y-values, and then connecting these points with a straight line.
• Techniques for graphing linear equations include plotting points, using the slope-intercept form, and determining intercepts. Each method aids in accurately representing the relationship between variables.

#### Flashcards in Linear graphs 12

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What is the slope of a linear graph?
The slope of a linear graph, also known as the gradient, is the measure of the steepness of the line. It is calculated as the ratio of the vertical change (rise) to the horizontal change (run) between two points on the line. It is usually represented as 'm' in the equation y = mx + c.
How do you find the y-intercept of a linear graph?
The y-intercept of a linear graph can be found by identifying the point where the line crosses the y-axis. In the equation \$$y = mx + c \$$, the y-intercept is the constant term \$$c \$$. It represents the value of \$$y \$$ when \$$x \$$ is zero.
How do you determine the equation of a linear graph?
To determine the equation of a linear graph, identify the slope (m) and the y-intercept (c). The slope is the change in y divided by the change in x between two points on the line. The y-intercept is the value of y when x is 0. The equation is then y = mx + c.
How do you plot a linear graph from a given equation?
To plot a linear graph from a given equation, identify the slope (gradient) and y-intercept from the equation in the form y = mx + c. Plot the y-intercept (c) on the y-axis. Use the slope (m) to determine another point by rising and running from the y-intercept. Draw a straight line through the points.
What are the properties of a linear graph?
The properties of a linear graph include a straight line appearance, a constant rate of change or slope, and it can be represented by the equation \$$y = mx + c \$$, where \$$m \$$ is the slope and \$$c \$$ is the y-intercept. The graph extends infinitely in both directions.

## Test your knowledge with multiple choice flashcards

What is the slope-intercept form of a linear equation?

Identify a common mistake in graphing linear equations.

What is a horizontal line represented by in equation form?

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