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The Slope of the Line y=mx+b

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Slope of a Line – Definition

In everyday life, we encounter slopes from the steepness of a hill to the angle of a ramp. In mathematics, the slope of a line measures how steep a line is. It is often also referred to as the gradient.

The slope represents the steepness or incline of a line, defined as the ratio of the vertical change (rise) to the horizontal change (run) between two points on the line.

21604_TheSlopeOfTheLine-01.svg

Understanding Slope as a Rate of Change

The slope of a line is a measure of how one variable changes in relation to another. It's the same as a rate of change you might encounter in everyday life, such as speed.

The slope of a line can be an integer (whole number) or a fraction or decimal too. It can be positive to represent a line going upwards from left to right, or negative indicating a line that goes down from left to right. It is given by the value of $m$ in the equation of a straight line, $y=mx+c$.

For example, imagine you're on a road trip and want to calculate the average speed of your journey. Speed is a rate of change - it reflects how distance changes over time. In this case, the slope (speed) is the change in distance (rise) divided by the change in time (run).

27728_TheSlopeOfTheLine-02.svg

Graphing the Data:

  • Plot these points on a graph with 'Time' on the $x$-axis and 'Distance' on the $y$-axis.

21604_TheSlopeOfTheLine-03.svg

Finding the Slope (Speed):

  • Choose any two points on the graph. For example, $(1, 50)$ and $(3, 150)$.
  • Apply the slope formula: Slope (Speed) = ($\dfrac{\text{rise}}{\text{run}}$) = ($\dfrac{\text{Change in Distance}}{\text{Change in Time}}$)
  • Slope (Speed) = $\frac{(150 - 50)}{(3 - 1)} = \frac{100}{2} = 50$ miles per hour.

Equation of the Line:

  • The equation representing this relationship in the form $y = mx$ could be written as Distance = $50 \times{time}$.
  • This equation tells us that for every hour of travel, the distance increases by 50 miles.

Understanding the concept of slope as a unit rate, like speed, helps make connections between mathematical concepts and real-world scenarios. It demonstrates how slope is not just a theoretical idea but a practical tool for everyday calculations.

Different Types of Slope

Type Of Slope Description Graph Example
Positive Slope Rises to the right, indicating the slope value is a positive number. As you move from left to right, the line goes up. 21604_TheSlopeOfTheLine-04.svg $m = \frac{3}{4}$
Negative Slope Falls to the right, indicating the slope value is a negative number. As you move from left to right, the line goes down. 21604_TheSlopeOfTheLine-05.svg $m = \frac{-5}{2}$
Zero Slope Represents a horizontal line, indicating the slope value is zero. There's no rise over the run. 21604_TheSlopeOfTheLine-17.svg $m = 0$
Undefined Slope Represents a vertical line, indicating the slope is undefined because division by zero is not possible in mathematics. 21604_TheSlopeOfTheLine-06.svg Slope is undefined

Finding the Slope on a Graph – Step-By-Step Instructions and Examples

When looking at a graph, the slope ($m$) is determined by how much the line rises (goes up or down) for every unit it runs (moves right). This can be visually represented and calculated as $m = \frac{\text{rise}}{\text{run}}$.

Step Description Visual
Identify Points Locate the points on the graph. For this example, the points are $(1, 1)$ and $(3, 4)$. 21604_TheSlopeOfTheLine-09.svg
Rise and Run Observe the rise and run between the points. From $1$ to $4$ is a rise of 3 units (upwards), and from $1$ to $3$ is a run of 2 units (to the right). 21604_TheSlopeOfTheLine-09.svg
Slope Calculation Calculate the slope using the rise over run. The slope $m = \frac{3}{2}$, indicating the line rises 3 units for every 2 units it moves to the right. 21604_TheSlopeOfTheLine-10.svg

Find the slope of the graphs.

21604_TheSlopeOfTheLine-11.svg

21604_TheSlopeOfTheLine-12.svg

21604_TheSlopeOfTheLine-13.svg

Finding the Slope When Given Two Points – Step-by-Step Instructions

To calculate the slope, use the formula $m = \frac{y_2 - y_1}{x_2 - x_1}$ when you know at least two points on the line. This formula makes it easy to see how steep a line is without a protractor.

What is the slope of the line that passes through the points $(6, -3)$ and $(2, 7)$?

To calculate the slope of a line given two points, $(x_1, y_1)$ and $(x_2, y_2)$, we use the slope formula:

21604_TheSlopeOfTheLine-14.svg

Step 1: Identify the coordinates of the two points.

Point 1: $(6, -3)$, where $x_1 = 6$ and $y_1 = -3$

Point 2: $(2, 7)$, where $x_2 = 2$ and $y_2 = 7$

Step 2: Plug the coordinates into the slope formula.

$m = \frac{7 - (-3)}{2 - 6}$

It is important to do the calculation in the correct order as mixing up the positions of the $x$ and $y$ values in the formula can result in different answers to the one we want!

Step 3: Perform the subtraction in the numerator and the denominator.

$m = \frac{7 + 3}{2 - 6}$

$m = \frac{10}{-4}$

Step 4: Simplify the fraction.

$m = -\frac{5}{2}$

Therefore, the slope of the line passing through the points $(6, -3)$ and $(2, 7)$ is $-\frac{5}{2}$.

This negative slope indicates that the line falls as it moves from left to right.

Try some on your own!

Computing the slope of a line can be useful when analysing data and identifying patterns.

Finding the Slope from an Equation

Understanding how to find the slope from an equation involves recognising the slope-intercept form of a line, which is expressed as $y = mx + c$. In this form, $m$ represents the slope of the line, indicating how steep it is and in which direction it tilts. The $c$ value represents the y-intercept, where the line crosses the y-axis.

The slope-intercept form $y = mx + c$ provides a straightforward way to identify the slope directly from the equation of a line. The coefficient of $x$ (that is, $m$) is the slope. This tells us by how many units $y$ changes for every one-unit increase in $x$.

Consider the equation of a line $y = 2x + 3$.

  • Identify the Slope: In the equation $y = 2x + 3$, the coefficient of $x$ is $2$. This means the slope ($m$) of the line is $2$.

  • Interpret the Slope: A slope of $2$ means that for every one unit the $x$ value increases, the $y$ value increases by $2$ units. This indicates the line rises to the right.

The Slope of a Line – Practice Questions

21604_TheSlopeOfTheLine-15.svg

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The Slope of a Line – Summary

Key Learnings from this Text:

  • The slope of a line tells us how steep it is by comparing the vertical change to the horizontal change between two points.

  • Slope is directly related to the rate of change, similar to everyday concepts like speed. It shows how quickly one thing changes in relation to another.

  • You can find the slope by using the formula $m = \frac{y_2 - y_1}{x_2 - x_1}$, which helps us understand the line's incline without needing any special tools.

  • By looking at a graph, we can visually determine the slope by identifying the rise and run between two points. This visual approach makes it easier to understand the concept of slope.

  • Understanding slope is crucial for solving real-life problems, from designing ramps to calculating how fast an object moves. It's a practical skill that connects classroom math to the world around us.

Having a solid understanding of the slope of a line will be helpful with graphing linear equations, and solving systems of equations by graphing.

The Slope of a Line – Frequently Asked Questions

The Slope of the Line y=mx+b exercise

Would you like to apply the knowledge you’ve learnt? You can review and practice it with the tasks for the learning text The Slope of the Line y=mx+b.
  • Hints

    The gradient or steepness of the straight line is measured by the difference in $y$ divided by the difference in $x$.

    We use the formal way to write it like this:

    $y = mx + c$

    • $m$ represents the gradient (steepness of the slope)
    • $c$ represents the $y$ intercept (where the line crosses the $y$ axis)

    We are looking for $m$, the coefficient of $x$.

    In this example, $y = 2x - 4$, the gradient $m = 2$

    Solution

    The answer is $3$.

    The line $\textbf{y = 3x + 6}$ is in the form $y = mx + c$, where $m$ is the gradient of the line and $c$ is the $y$-intercept. This is shown on the diagram.

  • Hints

    We are looking for the $y$-intercept. This is where the straight line cuts the $y$ axis.

    A straight line graph is written in the form:

    Sometimes it is written in a different order. For example, $y = 6 + 7x$, but the $y$-intercept is still the number by itself. The coefficient of $x$ is the gradient.

    In the straight line $y = 2x - 8$, the $y$ intercept is $-8$.

    Solution
    • $y = 2x + 3$, the $y$ intercept, $c = 3$.
    • $y = 3x - 2$, the $y$ intercept, $c = -2$.
    • $y = 4x - 3$, the $y$ intercept, $c = -3$.
    • $y = 4 + 2x$, the $y$ intercept, $c = 4$.
  • Hints

    To work out the equation of the line we use $y = mx + c$.

    • $m =$ gradient or steepness of the line.
    • $c =$ the $y$-intercept (where the line cuts the $y$ axis).

    To find the gradient, we use a triangle to find the change in $y$ divided by the change in $x$.

    The $y$-intercept it is where the line crosses through the $y$ axis.

    See the example.

    Solution

    $y = -2x + 3$

    • The $y$-intercept, $c$ is $3$.
    • the gradient, $m = \frac{-2}{1} = -2$.
  • Hints

    To work out the gradient rearrange the equation to the form $y = mx + c$.

    Rearrange the equation. For example,

    Solution

    Gradient $= 3$

    Rearrange the equation to the form $y = mx + c$.

    $m =$ gradient.

  • Hints

    The gradient is the steepness of the line.

    • Drop a triangle under the line.
    • Find the distance in $y$ divided by the distance in $x$.

    Here is an example of how to work out the gradient.

    • We draw a triangle under the line.
    • Change in $y = 6$.
    • Change in $x = 2$.
    • $\frac{6}{2} = 3$.

    For this question we could use the triangle shown.

    Divide the change in $y$ by the change in $x$ to get the gradient.

    Solution

    Gradient $= 2$.

    • We draw a triangle under the line.
    • Change in $y = 4$.
    • Change in $x = 2$.
    • $\frac{4}{2} = 2$.
  • Hints

    The equation must be rearranged into the form $y = mx + c$ in order to find the correct gradient, $m$ and correct $y$-intercept, $c$.

    For example,

    When the equation is in the form $y = mx + c$, we are looking for a graph which has a $y$-intercept of the value of $c$ and a gradient of the value of $m$.

    For example, $y = 6x + 7$ has a $y$-intercept of $7$ and a gradient of $6$.

    Solution

    $y = 2x + 1$

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