Illustrative Mathematics Grade 8, Unit 3, Lesson 13: More Solutions to Linear Equations

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Illustrative Math
Grade 8

Lesson 13: More Solutions to Linear Equations

Let’s find solutions to more linear equations.

Illustrative Math Unit 8.3, Lesson 13 (printable worksheets)

Lesson 13 Summary

Let’s think about the linear equation 2x - 4y = 12. If we know (0,-3) is a solution to the equation, then we also know (0, -3) is a point on the graph of the equation. Since this point is on the y-axis, we also know that it is the vertical intercept of the graph. But what about the coordinate of the horizontal intercept, when y = 0? Well, we can use the equation to figure it out.
2x - y = 12
2x - 4(0) = 12
2x = 12
x = 6
Since x = 6 when y = 0, we know the point (6, 0) is on the graph of the line. No matter the form a linear equation comes in, we can always find solutions to the equation by starting with one value and then solving for the other value.

Lesson 13.1 Coordinate Pairs

For each equation choose a value for x and then solve to find the corresponding y value that makes that equation true.

• See Video for Whole Lesson

Lesson 13.2 True or False: Solutions in the Coordinate Plane

Here are graphs representing three linear relationships. These relationships could also be represented with equations.
For each statement below, decide if it is true or false. Explain your reasoning.

1. (4, 0) is a solution of the equation for line m.
2. The coordinates of the point G make both the equation for line m and the equation for line n true.
3. x = 0 is a solution of the equation for line n.
4. (2, 0) makes both the equation for line m and the equation for line n true.
5. There is no solution for the equation for line l that has y = 0.
6. The coordinates of point H are solutions to the equation for line l.
7. There are exactly two solutions of the equation for line l.
8. There is a point whose coordinates make the equations of all three lines true.
   After you finish discussing the eight statements, find another group and check your answers against theirs. Discuss any disagreements.

Lesson 13.3 I’ll Take an X, Please

One partner has 6 cards labeled A through F and one partner has 6 cards labeled a through f. In each pair of cards (for example, Cards A and a), there is an equation on one card and a coordinate pair, (x, y), that makes the equation true on the other card.

1. The partner with the equation asks the partner with a solution for either the x-value or the y-value and explains why they chose the one they did.
2. The partner with the equation uses this value to find the other value, explaining each step as they go.
3. The partner with the coordinate pair then tells the partner with the equation if they are right or wrong. If they are wrong, both partners should look through the steps to find and correct any errors. If they are right, both partners move onto the next set of cards.
4. Keep playing until you have finished Cards A through F.

Are you ready for more?

Consider the equation ax + by = c, where a, b and c are positive numbers.

1. Find the coordinates of the x- and y-intercepts of the graph of the equation.
   • Show Answer

     The x-intercept is -c/a.
     The y-intercept is -c/b

2. Find the slope of the graph.
   • Show Answer

     The slope is -a/b

Lesson 13 Practice Problems

1. For each equation, find y when x = -3. Then find x when y = 2.
2. Match each graph of a linear relationship to a situation that most reasonably reflects its context.
3. True or false: The points (6, 13), (21, 33), and (99, 137) all lie on the same line. The equation of the line is y = 4/3 x + 5. Explain or show your reasoning.
4. Here is a linear equation: y = 1/4 x + 5/4
   a. Are (1, 1.5) and (12, 4) solutions to the equation? Explain or show your reasoning.
   b. Find the x-intercept of the graph of the equation. Explain or show your reasoning.

The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.

Try the free Mathway calculator and problem solver below to practice various math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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