Note: This activity can be completed by students in pairs or individually in front of computers.
Try this Task!
When is ax + b = bx + a? (Use + for addition, - for subtraction, * for multiplication, / for division, and ^ to raise an expression to a power.)
- In the Y= window, set y1 = ax + b and y2 = bx + a. Make sure to check the plot boxes at the left for both functions.
- Examine the graph of these functions. Change the values of the parameters a and b using the sliders. What do you notice?
- What conclusions can you make about the statement ax + b = bx + a?
Think about this Situation
Use the tool above to investigate this equation graphically and numerically for different values of a and b.
- What do you notice about the intersection of y1 = ax + b and y2 = bx + a?
- What does this tell you about the equation ax + b = bx + a?
- What can you say about the equation ax + b = bx - a?
- What other similar questions could be explored using three parameters a, b, and c?
Discussion
This investigation has focused on equations as statements of numeric equality or inequality between two objects. The progression of tasks moves from a concrete notion to thinking of an equation as stating a relationship between two symbolic expressions and how this relationship can be investigated using graphical or numerical representations. Of course, this investigation illuminates only a portion of the role of equivalence in mathematics.
The equals sign has many other uses and interpretations. Each use provides an alternative viewpoint on the concept of equality, a different way in which mathematical objects can be equivalent. For example, the equals sign is used:
- defining functions, such as f(x) = x + 1
- in assignments such as I = I + 1 in computer programming
- in creating structures such as x2 + y2 = r2 (circle)
- in creating equivalence classes such as 5 = 17 (mod 12).
