Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Elementary Algebra 2e

Key Concepts

Elementary Algebra 2eKey Concepts
Search for key terms or text.

Key Concepts

4.1 Use the Rectangular Coordinate System

  • Sign Patterns of the Quadrants
     Quadrant IQuadrant IIQuadrant IIIQuadrant IV(x,y)(x,y)(x,y)(x,y)(+,+)(−,+)(−,−)(+,−)Quadrant IQuadrant IIQuadrant IIIQuadrant IV(x,y)(x,y)(x,y)(x,y)(+,+)(−,+)(−,−)(+,−)
  • Points on the Axes
    • On the x-axis, y=0y=0. Points with a y-coordinate equal to 0 are on the x-axis, and have coordinates (a,0)(a,0).
    • On the y-axis, x=0x=0. Points with an x-coordinate equal to 0 are on the y-axis, and have coordinates (0,b).(0,b).
  • Solution of a Linear Equation
    • An ordered pair (x,y)(x,y) is a solution of the linear equation Ax+By=CAx+By=C, if the equation is a true statement when the x- and y- values of the ordered pair are substituted into the equation.

4.2 Graph Linear Equations in Two Variables

  • Graph a Linear Equation by Plotting Points
    1. Step 1. Find three points whose coordinates are solutions to the equation. Organize them in a table.
    2. Step 2. Plot the points in a rectangular coordinate system. Check that the points line up. If they do not, carefully check your work!
    3. Step 3. Draw the line through the three points. Extend the line to fill the grid and put arrows on both ends of the line.

4.3 Graph with Intercepts

  • Find the x- and y- Intercepts from the Equation of a Line
    • Use the equation of the line to find the x- intercept of the line, let y=0y=0 and solve for x.
    • Use the equation of the line to find the y- intercept of the line, let x=0x=0 and solve for y.
  • Graph a Linear Equation using the Intercepts
    1. Step 1. Find the x- and y- intercepts of the line.
       Let y=0y=0 and solve for x.
       Let x=0x=0 and solve for y.
    2. Step 2. Find a third solution to the equation.
    3. Step 3. Plot the three points and then check that they line up.
    4. Step 4. Draw the line.



  • Strategy for Choosing the Most Convenient Method to Graph a Line:
    • Consider the form of the equation.
    • If it only has one variable, it is a vertical or horizontal line.
       x=ax=a is a vertical line passing through the x- axis at aa
       y=by=b is a horizontal line passing through the y- axis at bb.
    • If y is isolated on one side of the equation, graph by plotting points.
    • Choose any three values for x and then solve for the corresponding y- values.
    • If the equation is of the form ax+by=cax+by=c, find the intercepts. Find the x- and y- intercepts and then a third point.

4.4 Understand Slope of a Line

  • Find the Slope of a Line from its Graph using m=riserunm=riserun
    1. Step 1. Locate two points on the line whose coordinates are integers.
    2. Step 2. Starting with the point on the left, sketch a right triangle, going from the first point to the second point.
    3. Step 3. Count the rise and the run on the legs of the triangle.
    4. Step 4. Take the ratio of rise to run to find the slope.



  • Graph a Line Given a Point and the Slope
    1. Step 1. Plot the given point.
    2. Step 2. Use the slope formula m= rise run m= rise run to identify the rise and the run.
    3. Step 3. Starting at the given point, count out the rise and run to mark the second point.
    4. Step 4. Connect the points with a line.



  • Slope of a Horizontal Line
    • The slope of a horizontal line, y=by=b, is 0.
  • Slope of a vertical line
    • The slope of a vertical line, x=ax=a, is undefined

4.5 Use the Slope-Intercept Form of an Equation of a Line

  • The slope–intercept form of an equation of a line with slope mm and y-intercept, (0,b)(0,b) is, y=mx+by=mx+b.
  • Graph a Line Using its Slope and y-Intercept
    1. Step 1. Find the slope-intercept form of the equation of the line.
    2. Step 2. Identify the slope and y-intercept.
    3. Step 3. Plot the y-intercept.
    4. Step 4. Use the slope formula m=riserunm=riserun to identify the rise and the run.
    5. Step 5. Starting at the y-intercept, count out the rise and run to mark the second point.
    6. Step 6. Connect the points with a line.



  • Strategy for Choosing the Most Convenient Method to Graph a Line: Consider the form of the equation.
    • If it only has one variable, it is a vertical or horizontal line.
      x=ax=a is a vertical line passing through the x-axis at aa.
      y=by=b is a horizontal line passing through the y-axis at bb.
    • If yy is isolated on one side of the equation, in the form y=mx+by=mx+b, graph by using the slope and y-intercept.
       Identify the slope and y-intercept and then graph.
    • If the equation is of the form Ax+By=CAx+By=C, find the intercepts.
       Find the x- and y-intercepts, a third point, and then graph.
  • Parallel lines are lines in the same plane that do not intersect.
    • Parallel lines have the same slope and different y-intercepts.
    • If m1 and m2 are the slopes of two parallel lines then m1=m2.m1=m2.
    • Parallel vertical lines have different x-intercepts.
  • Perpendicular lines are lines in the same plane that form a right angle.
    • If m1andm2m1andm2 are the slopes of two perpendicular lines, then m1·m2=−1m1·m2=−1 and m1=−1m2m1=−1m2.
    • Vertical lines and horizontal lines are always perpendicular to each other.

4.6 Find the Equation of a Line

  • To Find an Equation of a Line Given the Slope and a Point
    1. Step 1. Identify the slope.
    2. Step 2. Identify the point.
    3. Step 3. Substitute the values into the point-slope form, y−y1=m(x−x1)y−y1=m(x−x1).
    4. Step 4. Write the equation in slope-intercept form.



  • To Find an Equation of a Line Given Two Points
    1. Step 1. Find the slope using the given points.
    2. Step 2. Choose one point.
    3. Step 3. Substitute the values into the point-slope form, y−y1=m(x−x1)y−y1=m(x−x1).
    4. Step 4. Write the equation in slope-intercept form.



  • To Write and Equation of a Line
    • If given slope and y-intercept, use slope–intercept form y=mx+by=mx+b.
    • If given slope and a point, use point–slope form y−y1=m(x−x1)y−y1=m(x−x1).
    • If given two points, use point–slope form y−y1=m(x−x1)y−y1=m(x−x1).



  • To Find an Equation of a Line Parallel to a Given Line
    1. Step 1. Find the slope of the given line.
    2. Step 2. Find the slope of the parallel line.
    3. Step 3. Identify the point.
    4. Step 4. Substitute the values into the point-slope form, y−y1=m(x−x1)y−y1=m(x−x1).
    5. Step 5. Write the equation in slope-intercept form.



  • To Find an Equation of a Line Perpendicular to a Given Line
    1. Step 1. Find the slope of the given line.
    2. Step 2. Find the slope of the perpendicular line.
    3. Step 3. Identify the point.
    4. Step 4. Substitute the values into the point-slope form, y−y1=m(x−x1)y−y1=m(x−x1).
    5. Step 5. Write the equation in slope-intercept form.

4.7 Graphs of Linear Inequalities

  • To Graph a Linear Inequality
    1. Step 1. Identify and graph the boundary line.
       If the inequality is ≤or≥≤or≥, the boundary line is solid.
       If the inequality is < or >, the boundary line is dashed.
    2. Step 2. Test a point that is not on the boundary line. Is it a solution of the inequality?
    3. Step 3. Shade in one side of the boundary line.
       If the test point is a solution, shade in the side that includes the point.
       If the test point is not a solution, shade in the opposite side.
PreviousNext
Order a print copy
Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:

    Access for free at https://openstax.org/books/elementary-algebra-2e/pages/1-introduction

  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:

    Access for free at https://openstax.org/books/elementary-algebra-2e/pages/1-introduction

Citation information

© Jul 8, 2025 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.