4.11: Direct Variation
Suppose a person's annual salary is directly proportional to the number of years he or she has spent in school. What do you think this means? Could you determine the constant of proportionality? What information would you need to do so? If you wrote a linear equation to represent this relationship, what would be the slope and \begin{align*}y\end{align*}
Direct Variation
At the local farmer’s market, you saw someone purchase 5 pounds of strawberries and pay $12.50. You want to buy strawberries too, but you want only 2 pounds. How much would you expect to pay?
This situation is an example of a direct variation. You would expect that the strawberries are priced on a “per pound” basis, and that if you buy twofifths of the amount of strawberries, you would pay twofifths of $12.50 for your strawberries, or $5.00. Similarly, if you bought 10 pounds of strawberries (twice the amount), you would pay $25.00 (twice $12.50), and if you did not buy any strawberries you would pay nothing.
Direct variation can be expressed as the equation \begin{align*}y=(k)x\end{align*}
Direct variation occurs when:
 The fraction \begin{align*}\frac{rise}{run}\end{align*}
riserun or \begin{align*}\frac{change \ in \ y}{change \ in \ x}\end{align*}change in ychange in x is always the same, and  The ordered pair (0, 0) is a solution to the equation.
Example A
If \begin{align*}y\end{align*}
Solution: We can solve for the constant of proportionality using substitution.
Substitute \begin{align*}x=2.5\end{align*}
\begin{align*}7.5 & = k(2.5) && \text{Divide both sides by} \ 2.5.\\
\frac{7.5}{2.5}& =k=3\end{align*}
The constant of variation (or the constant of proportionality) is 3.
You can use this information to graph this direct variation situation. Remember that all direct variation situations cross the origin. You can plot the ordered pair (0, 0) and use the constant of variation as your slope.
Example B
Explain why each of the following equations are not examples of direct variation.
\begin{align*}y& =\frac{2}{x}\\
y& =5x1\\
2x+y& =6\end{align*}
Solution: In equation 1, the variable is in the denominator of the fraction, violating the definition.
In equation 2, there is a \begin{align*}y\end{align*}
In equation 3, there is also a \begin{align*}y\end{align*}
Translating a Sentence into a Direct Variation Equation
Direct variation equations use the same phrase to give the reader a clue. The phrase is either “directly proportional” or “varies directly.”
Example C
The area of a square varies directly as the square of its side.
Solution: The first variable you encounter is “area.” Think of this as your \begin{align*}y\end{align*}
Now translate into an equation: \begin{align*}y=(k)×s^2\end{align*}
You’ve written your first direct variation equation.
Guided Practice
The distance you travel is directly proportional to the time you have been traveling. Write this situation as a direct variation equation.
Solution:
The first variable is distance; call it \begin{align*}d\end{align*}
\begin{align*}d=(k) \times t\end{align*}
Practice
Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK12 Basic Algebra: Direct Variation Models (11:11)
 Describe direct variation.
 What is the formula for direct variation? What does \begin{align*}k\end{align*}
k represent?
Translate the following direct variation situations into equations. Choose appropriate letters to represent the varying quantities.
 The amount of money you earn is directly proportional to the number of hours you work.
 The weight of an object on the Moon varies directly with its weight on Earth.
 The volume of a gas is directly proportional to its temperature in Kelvin.
 The number of people served varies directly with the amount of ground meat used to make burgers.
 The amount of a purchase varies directly with the number of pounds of peaches.
Explain why each equation is not an example of direct variation.

\begin{align*}\frac{4}{x}=y\end{align*}
4x=y 
\begin{align*}y=9\end{align*}
y=9 
\begin{align*}x=3.5\end{align*}
x=−3.5 
\begin{align*}y=\frac{1}{8} x+7\end{align*}
y=18x+7 
\begin{align*}4x+3y=1\end{align*}
4x+3y=1
Graph the following direct variation equations.

\begin{align*}y=\frac{4}{3}x\end{align*}
y=43x 
\begin{align*}y=\frac{2}{3}x\end{align*}
y=−23x 
\begin{align*}y=\frac{1}{6}x\end{align*}
y=−16x 
\begin{align*}y=1.75x\end{align*}
y=1.75x  Is \begin{align*}y=6x2\end{align*}
y=6x−2 an example of direct variation? Explain your answer.
Mixed Review
 Graph \begin{align*}3x+4y=48\end{align*}
3x+4y=48 using its intercepts.  Graph \begin{align*}y=\frac{2}{3} x4\end{align*}
y=23x−4 .  Solve for \begin{align*}u: 4(u+3)=3(3u7)\end{align*}
u:4(u+3)=3(3u−7) .  Are these lines parallel? \begin{align*}y=\frac{1}{2} x7\end{align*}
y=12x−7 and \begin{align*}2y=x+2\end{align*}2y=x+2  In which quadrant is (–99, 100)?
 Find the slope between (2, 0) and (3, 7).
 Evaluate if \begin{align*}a=3\end{align*}
a=−3 and \begin{align*}b=4\end{align*}b=4 : \begin{align*}\frac{1+4b}{2a5b}\end{align*}1+4b2a−5b .
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Here you'll learn what direct variation means and how to calculate a constant of proportionality.