You and your students learned about linear equations in Units I and 3 in MathSteps. Now they will review how to solve those equations and learn how to graph linear equations in for x and y.
Materials: overhead transparency or poster paper
Preparation: Draw a coordinate grid on a transparency or poster paper.
Prerequisite Skills and Concepts: Students should know how to graph ordered pairs. They should also be able to solve simple addition, subtraction, multiplication, and division linear equations.
Write the equation x 3 = y on the board.
- Ask: Who could tell me what this equation means in words?
Students should say that "one number minus 3 is equal to another number," or something equivalent.
- Say: Earlier in the year we worked with similar equations, but those equations only had one unknown, an x or a y. This equation has two unknowns, x and y. If I were to let x = 2 in this equation, what would the value of y be?
Students will calculate that y = -1. Draw a table with four columns. Label the first column x, the second column x 3 = y, the third column y, and the fourth column (x, y). Put 2, 2 3 = -1, -1, and (2, -1) in the appropriate columns of the first row.
- Say: Since y = -1 when x = 2, I put 2 in the x-column, 2 3 = -1 in the x 3 = y, -1 in the y-column, and the ordered pair (2, -1) in the (x, y) column.
Solicit 4 or 5 other values for x and find their corresponding values for y. List them in the table accordingly.
- Say: We now have two ways to represent the relationship between the numbers x and y. The equation describes a general relationship between the numbers, which in words says that one number minus 3 is equal to another number. The table now gives us some specific values which satisfy that relationship. If you look at the table, you will see that every y-value is 3 less than its corresponding x-value.
- Say: Now we are going to look at a third way to represent the equation x 3 = y. This way is a visual representation using a graph. We are going to use the values from our table and graph the ordered pairs on the graph.
- Ask: Using this graph (the overhead or large poster graph), who could show us where the point (2, -1) is located?
Have a student graph the point on the grid. Do the same for the other 4 or 5 points in the table.
- Ask: What do you notice about the points we graphed?
Students will respond that they appear to lie in a straight line. If they don't, show them by drawing a line through all the points. Make sure you indicate that the line goes on indefinitely in both directions by drawing arrows on the ends of the lines.
- Say: Let's try one more equation. (Write the equation 2x = y on the board.) How are the two variables in this equation related?
Students should respond that the y-value is double the x-value.
- Ask: If we want to graph this equation, what should we do next?
Students will say to create a table of values for x and y. So, create a table of values with at least 5 points in it.
- Ask: Now what do we do?
Graph the points on a grid. Have volunteers plot the points from the table of values on the coordinate grid for the class to see..
- Ask: What do you notice about these values?
Students should say that they too seem to lie in a straight line. Draw the double arrowhead line on the grid.
Graphing Linear Equations