[PRIORITY] Which Variable to Solve for? (Part 1)

5 min

Narrative

In this Warm-up, students look for a relationship between two quantities by interpreting a verbal description and analyzing pairs of values in a table. They then use the observed relationship to find unknown values of one quantity given the other and to think about possible equations that could represent the relationship more generally (MP8).

The work here reinforces the idea that the relationship between two quantities can be expressed in more than one way, and that some forms might be more helpful than others, depending on what we want to know. In this context, for instance, if we know the area of the parallelogram and want to know its base length, the equation b=A3b = \frac{A}{3} is more helpful than A=3bA=3b.

Launch

Give students access to four-function calculators, if requested.

Student Task

  1. The table shows the relationship between the base length, bb, and the area, AA, of some parallelograms. All the parallelograms have the same height. Base length is measured in inches, and area is measured in square inches. Complete the table.
    bb (inches) AA (square inches)
    1 3
    2 6
    3 9
    4.5  
    112\frac{11}{2}  
      36
      46.5
  2. Decide whether each equation could represent the relationship between bb and AA. Be prepared to explain your reasoning.

    1. b=3Ab = 3A
    2. b=A3b = \frac {A}{3}
    3. A=b3A = \frac {b}{3}
    4. A=3bA = 3b

Sample Response

  1.  
    bb (inches) AA (square inches)
    1 3
    2 6
    3 9
    4.5 13.5
    112\frac{11}{2} 332\frac{33}{2}
    12 36
    15.5 46.5
    1. No
    2. Yes
    3. No
    4. Yes

Synthesis

Invite students to share their responses and explanations. Then focus the whole-class discussion on the second question. Discuss with students:

  • "Are the two equations we chose equivalent? How do you know?" (Yes. There is an acceptable move that takes one to the other. If we divide each side of A=3bA=3b by 3, we have A3=b\frac{A}{3}=b, which can also be written as b=A3b=\frac{A}{3}. If we multiply each side of b=A3b=\frac{A}{3} by 3, we have 3b=A3b=A or A=3bA = 3b.)
  • "If we know the base, which equation would make it easier to find the area? Why?" (A=3bA=3b. The variable for area is already isolated. All we have to do is multiply the base by 3 to find the area.)
  • "If we know the area, which equation would make it easier to find the base? Why?" (b=A3b=\frac{A}{3}. The variable for the base is already isolated. We can just divide the area by 3 to find the base.)
Anticipated Misconceptions

Some students may think that the height must be known before they could find the missing area or base. Encourage them to look for a pattern in the table and to reason from there.

Standards
Building Toward
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • A-CED.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <em>For example, rearrange Ohm's law V = IR to highlight resistance R.</em>
  • HSA-CED.A.4·Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. <span>For example, rearrange Ohm's law <span class="math">\(V = IR\)</span> to highlight resistance <span class="math">\(R\)</span>.</span>

15 min

15 min