Lesson 5
Comparing Relationships with Equations
Let’s develop methods for deciding if a relationship is proportional.
5.1: Notice and Wonder: Patterns with Rectangles
Do you see a pattern? What predictions can you make about future rectangles in the set if your pattern continues?
5.2: More Conversions
The other day you worked with converting meters, centimeters, and millimeters. Here are some more unit conversions.
- Use the equation \(F =\frac95 C + 32\), where \(F\) represents degrees Fahrenheit and \(C\) represents degrees Celsius, to complete the table.
temperature \((^\circ\text{C})\) temperature \((^\circ\text{F})\) 20 4 175 - Use the equation \(c = 2.54n\), where \(c\) represents the length in centimeters and \(n\) represents the length in inches, to complete the table.
length (in) length (cm) 10 8 3\(\frac12\) - Are these proportional relationships? Explain why or why not.
5.3: Total Edge Length, Surface Area, and Volume
Here are some cubes with different side lengths. Complete each table. Be prepared to explain your reasoning.
- How long is the total edge length of each cube?
side
lengthtotal
edge length3 5 \(9\frac12\) \(s\) - What is the surface area of each cube?
side
lengthsurface
area3 5 \(9\frac12\) \(s\) - What is the volume of each cube?
side
lengthvolume 3 5 \(9\frac12\) \(s\) - Which of these relationships is proportional? Explain how you know.
-
Write equations for the total edge length \(E\), total surface area \(A\), and volume \(V\) of a cube with side length \(s\).
- A rectangular solid has a square base with side length \(\ell\), height 8, and volume \(V\). Is the relationship between \(\ell\) and \(V\) a proportional relationship?
- A different rectangular solid has length \(\ell\), width 10, height 5, and volume \(V\). Is the relationship between \(\ell\) and \(V\) a proportional relationship?
- Why is the relationship between the side length and the volume proportional in one situation and not the other?
5.4: All Kinds of Equations
Here are six different equations.
\(y = 4 + x\)
\(y = \frac{x}{4}\)
\(y = 4x\)
\(y = 4^{x}\)
\(y = \frac{4}{x}\)
\(y = x^{4}\)
- Predict which of these equations represent a proportional relationship.
- Complete each table using the equation that represents the relationship.
- Do these results change your answer to the first question? Explain your reasoning.
- What do the equations of the proportional relationships have in common?
Summary
If two quantities are in a proportional relationship, then their quotient is always the same. This table represents different values of \(a\) and \(b\), two quantities that are in a proportional relationship.
\(a\) | \(b\) | \(\frac{b}{a}\) |
---|---|---|
20 | 100 | 5 |
3 | 15 | 5 |
11 | 55 | 5 |
1 | 5 | 5 |
Notice that the quotient of \(b\) and \(a\) is always 5. To write this as an equation, we could say \(\frac{b}{a}=5\). If this is true, then \(b=5a\). (This doesn’t work if \(a=0\), but it works otherwise.)
If quantity \(y\) is proportional to quantity \(x\), we will always see this pattern: \(\frac{y}{x}\) will always have the same value. This value is the constant of proportionality, which we often refer to as \(k\). We can represent this relationship with the equation \(\frac{y}{x} = k\) (as long as \(x\) is not 0) or \(y=kx\).
Note that if an equation cannot be written in this form, then it does not represent a proportional relationship.