5.5 - Lesson - Graphing Linear Inequalities Video Lesson



How to Graph Inequalities

Three Methods:

You can graph a linear or quadratic inequality similarly to how you would graph an equation. The difference is that, since an inequality shows a set of values greater than or less than, your graph will show more than just a dot on a number line or a line on a coordinate plane. By using algebra and evaluating the inequality sign, you can determine which values are included in the solution of an inequality.

Steps

Graphing a Linear Inequality on a Number Line

  1. Solve for the variable.To solve the inequality isolate the variable using the same algebraic methods you would use to solve an equation.Remember that when you multiply or divide by a negative number, you need to flip the inequality sign.
    • For example, if you are solving the inequality3y+9>12{\displaystyle 3y+9>12}, isolate the variable by subtracting 9 from each side of the inequality, then dividing by 3:
      3y+9>12{\displaystyle 3y+9>12}
      3y+9−9>12−9{\displaystyle 3y+9-9>12-9}
      3y>3{\displaystyle 3y>3}
      3y3>33{\displaystyle {\frac {3y}{3}}>{\frac {3}{3}}}
      y>1{\displaystyle y>1}
    • Your inequality should only have one variable. If your inequality has two variables, it is more appropriate to graph it on a coordinate plane using another method.
  2. Draw a number line.Include the relative value on your number line (the value you found the variable to be less than, greater than, or equal to). Make the number line as long or short as required.
    • For example, if you found thaty>1{\displaystyle y>1}, make sure to include a point for 1 on the number line.
  3. Draw a circle indicating the relative value.If the value is less than (<{\displaystyle <}) or greater than (>{\displaystyle >}) this number, the circle should be open, since the solution does not include the value. If the value is less than or equal to (≤{\displaystyle \leq }), or greater than or equal to (≥{\displaystyle \geq }), the circle should be filled in, since the solution includes the value.
    • For example, ify>1{\displaystyle y>1}, you would draw a circle at 1 on the number line. You would not fill in the circle, since 1 is not included in the solution.
  4. Draw an arrow indicating the included values.If the variable is greater than the relative value, your arrow should point to the right, since the solution includes values greater than that number. If the variable is less than the relative value, your arrow should point to the left, since the solution includes values less than that number.
    • For example, for the solutiony>1{\displaystyle y>1}, you would draw an arrow pointing to the right, since the solution includes values greater than 1.

Graphing a Linear Inequality on a Coordinate Plane

  1. Solve fory{\displaystyle y}.You want to find the equation of the line, so to do this you need to isolate they{\displaystyle y}variable on the left side of the equation using algebra.The right side of the equation should have thex{\displaystyle x}variable, and likely, a constant.
    • For example, for the inequality3y+9>9x{\displaystyle 3y+9>9x}, you would isolate the y variable by subtracting 9 from both sides, then dividing by 3:
      3y+9>9x{\displaystyle 3y+9>9x}
      3y+9−9>9x−9{\displaystyle 3y+9-9>9x-9}
      3y>9x−9{\displaystyle 3y>9x-9}
      3y3>9x−93{\displaystyle {\frac {3y}{3}}>{\frac {9x-9}{3}}}
      y>3x−3{\displaystyle y>3x-3}
  2. Graph the line on a coordinate plane.To do this, turn the inequality into an equation, and graph as you would any equation of a line.Plot the y-intercept, then use the slope to graph other points on the line.
    • For example, if the inequality isy>3x−3{\displaystyle y>3x-3}, you would graph the liney=3x−3{\displaystyle y=3x-3}. The y-intercept (the point where the line crosses the y axis) is -3, and the slope is 3, or31{\displaystyle {\frac {3}{1}}}. So, you would draw a point at(0,−3){\displaystyle (0,-3)}. The point above the y-intercept is(1,0){\displaystyle (1,0)}. The point below the y-intercept is(−1,−6){\displaystyle (-1,-6)}.
  3. Draw the line.If the inequality is less than (<{\displaystyle <}) or greater than (>{\displaystyle >}), the line should be dashed, since the solution does not include values equal to the line. If the value is less than or equal to (≤{\displaystyle \leq }), or greater than or equal to (≥{\displaystyle \geq }), the line should be solid, since the solution includes values equal to the line.
    • For example, since the inequality isy>3x−3{\displaystyle y>3x-3}, the line should be dashed, since the values do not include points on the line.
  4. Shade in the appropriate area.If the inequality showsy>mx+b{\displaystyle y>mx+b}you should shade in the area above the line. If the inequality showsy<mx+b{\displaystyle y<mx+b}, you should shade the area below the line.
    • For example, for the inequalityy>3x−3{\displaystyle y>3x-3}you would shade above the line.

Graphing a Quadratic Inequality on a Coordinate Plane

  1. Determine whether you have a quadratic inequality.A quadratic inequality takes the form ofax2+bx+c{\displaystyle ax^{2}+bx+c}.Sometimes there may not be anx{\displaystyle x}term or a constant, but there should always be anx2{\displaystyle x^{2}}term on one side of the inequality, and an isolatedy{\displaystyle y}variable on the other side.
    • For example, you might need to graph the inequalityy<x2−10x+16{\displaystyle y<x^{2}-10x+16}.
  2. Graph the line on a coordinate plane.To do this, turn the inequality into an equation, and graph the line as you normally would. Since you have a quadratic equation, the line will be a parabola.
    • For example, for the inequalityy<x2−10x+16{\displaystyle y<x^{2}-10x+16}, you would graph the liney=x2−10x+16{\displaystyle y=x^{2}-10x+16}. The vertex is at point(5,−9){\displaystyle (5,-9)}, and the parabola crosses the x-axis at the points(2,0){\displaystyle (2,0)}and(8,0){\displaystyle (8,0)}.
  3. Draw the parabola.Draw the parabola with a dashed line if the inequality is less than (<{\displaystyle <}) or greater than (>{\displaystyle >}). If the value is less than or equal to (≤{\displaystyle \leq }), or greater than or equal to (≥{\displaystyle \geq }), you should draw the parabola with a solid line, since the solution includes values equal to the line.
    • For example, for the inequalityy<x2−10x+16{\displaystyle y<x^{2}-10x+16}, you would draw the parabola with a dashed line.
  4. Find some test points.In order to determine which area to shade, you need to pick points from inside the parabola, and from outside the parabola.
    • For example, the graph of the inequalityy<x2−10x+16{\displaystyle y<x^{2}-10x+16}shows that the point(0,0){\displaystyle (0,0)}is outside the parabola. This would be a good point to use to test the solution.
  5. Shade the appropriate area.To determine which area to shade, plug the values ofx{\displaystyle x}andy{\displaystyle y}from your test points into the original inequality. Whichever point produces a true inequality indicates which area of the graph should be shaded in.
    • For example, plugging the values ofx{\displaystyle x}andy{\displaystyle y}of the point(0,0){\displaystyle (0,0)}into the original inequality, you get:
      y<x2−10x+16{\displaystyle y<x^{2}-10x+16}
      0<02−0x+16{\displaystyle 0<0^{2}-0x+16}
      0<16{\displaystyle 0<16}
      Since this is true, you would shade the area of the graph where the point(0,0){\displaystyle (0,0)}is found. In this case, this is outside of the parabola, not inside of it.

Community Q&A

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  • Question
    How do I graph the inequality y is less than or equal to X + 6?
    wikiHow Contributor
    Community Answer
    Since you do not have an x-squared term, you are working with a linear inequality. Turn the inequality into the equation y = x + 6. Plot the line using this equation. Since it is in the form of y = mx + b, you can tell that the y-intercept is at the point (0, 6), and that the slope is 1, or 1/1. Since it is less than OR equal to, the line should be solid. Since y is less than the line, you would shade the area below the line.
    Thanks!
  • Question
    How do I graph x - y is greater than 5 as a linear inequality?
    wikiHow Contributor
    Community Answer
    First, substitute "greater than" with "equals," which gives you x - y = 5. Then rearrange the equation to get y = x - 5. Input some values for x. If you do 1, 5, 10, you get y-values -4, 0, 5. Plot these 3 points (1,-4), (5,0) and (10,5). Then pick a point on your graph (not on the line) and put this into your starting equation. Take the point (4,2) for example. You have 4 - 2 > 5, and 2 > 5 is false. This means that the graph area on the same side of the line as point (4,2) is not in the region x - y > 5. Therefore, any point on the other side of the line should be in this region. To check, use point (8,2). 8 - 2 > 5, 6 > 5, which is true. You can then draw arrows off your line indicating the region represented by x - y > 5.
    Thanks!
  • Question
    How do I graph y=5x+1?
    wikiHow Contributor
    Community Answer
    The Y-axis goes up and down; the X-axis goes left and right. Plug in x values: In the equation above, if x is 0, what is y? Plugging 0 in for x and doing the calculation gives a 1 for y. The first point, then, is (0,1). (When x is 0, y is 1.) Next, plug in 1 for x. (5 times 1) + 1 = 6. So the next point is (1,6). (When x is 1, y is 6.) Now, plug in 2 for x. (5 times 2) + 1 = 11. The next point is (2,11). Plot the points on the graph and draw a line through all of them.
    Thanks!
  • Question
    How do I graph inequality equation -4x + 2 is less than or equal to 10?
    wikiHow Contributor
    Community Answer
    Inequalities should be graphed on a number line. Solve the equation algebraically, and plot it on the line with a dot on the answer. The dot should be open, with an arrow pointing toward the left.
    Thanks!
  • Question
    How do I graph a no solution?
    Top Answerer
    A graph of "no solution" would typically be two graph lines which never intersect.
    Thanks!
Unanswered Questions
  • Can you draw a graph for X+Y greater than or = 9?
  • How to plot a graph of T=3(x+1) for -2 is less than or equal to x and x is less than or equal to 3?
  • How do you find the answer to y
  • How do I draw a graph when the range is known?
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  • Always simplify the inequalities before graphing them.
  • If you get really stuck, you can input the inequality into a graphing calculator and try to work backwards.

Video

Quick Summary

To graph linear inequalities, isolate the variable and solve the problem. Then, draw a vertical or horizontal number line that includes the value that the variable is greater or less than, and draw a circle on the tick mark of the relative value. Draw an arrow from that value to the greater or less than side of the number line, depending on the equation. For example, for an equation that solves as y > 1, you would draw a circle on 1 and an arrow pointing up, since the solution includes all values greater than 1.

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Video: Graphing a linear inequality by the x and y intercepts

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Date: 19.12.2018, 14:26 / Views: 41341