Two variables vary directly if they are related by an equation of the form $y = kx$, where $k$ is a real number known as the constant of proportionality.

A
The

**direct variation**equation is a relationship between two variables that can be written in the form:$y=kx$

**constant of proportionality**is the coefficient and nonzero constant $k$ in the direct variation equation. In a direct variation relationship, one variable is proportional to the other. This means that one variable changes in proportion to, or by the same factor as, the other. So, when the value of $x$ is multiplied by $k$, the value of $y$ is multiplied by $k$. The graph of a direct variation relationship $y=kx$ is a line that passes through the origin, $(0,0)$, and the point $(1,k)$. The value of $k$ is the slope of the line, and $y$ is proportional to $x$. The $y$-intercept is zero because the line passes through the origin.Step-By-Step Example

Graphing a Direct Variation Equation

Identify the coefficient of proportionality and then graph the direct variation:

$y = \frac{3}{4}x$

Step 1

Use the direct variation equation, where $k$ is the constant of proportionality.
The constant of proportionality in the given equation is $\frac{3}{4}$.

$\begin{aligned}y&=kx\\y&=\frac{3}{4}x\\ \end{aligned}$

Step 2

The graph of a direct variation passes through the point $(0,0)$ because $b$ is zero.

The constant of proportionality is the slope of the line:$\frac{\text{Rise}}{\text{Run}}=\frac{3}{4}$

Solution

Use the slope to determine a second point on the line. Start at the origin and move up 3 units and right 4 units. Then draw a line through the points.

The graph is a line through the origin with a slope of $\frac{3}{4}$. Each time the $x$-value increases by 3 units, the $y$-value increases by 4 units.