2.2 Graphs of linear functions  (Page 3/15)

Graphing a function using transformations

Another option for graphing is to use transformations of the identity function $f(x)=x$ . A function may be transformed by a shift up, down, left, or right. A function may also be transformed using a reflection, stretch, or compression.

Vertical stretch or compression

In the equation $f(x)=mx,$ the $m$ is acting as the vertical stretch    or compression of the identity function. When $m$ is negative, there is also a vertical reflection of the graph. Notice in [link] that multiplying the equation of $f(x)=x$ by $m$ stretches the graph of $f$ by a factor of $m$ units if $m>\text{1}$ and compresses the graph of $f$ by a factor of $m$ units if $0<m<1.$ This means the larger the absolute value of $m,$ the steeper the slope.

Vertical shift

In $f(x)=mx+b,$ the $b$ acts as the vertical shift    , moving the graph up and down without affecting the slope of the line. Notice in [link] that adding a value of $b$ to the equation of $f\left(x\right)=x$ shifts the graph of $f$ a total of $b$ units up if $b$ is positive and $\left|b\right|$ units down if $b$ is negative.

Using vertical stretches or compressions along with vertical shifts is another way to look at identifying different types of linear functions. Although this may not be the easiest way to graph this type of function, it is still important to practice each method.

Graphing by using transformations

Graph $f(x)=\frac{1}{2}x-3$ using transformations.

The equation for the function shows that $m=\frac{1}{2}$ so the identity function is vertically compressed by $\frac{1}{2}.$ The equation for the function also shows that $b=\mathrm{-3}$ so the identity function is vertically shifted down 3 units. First, graph the identity function, and show the vertical compression as in [link] .

Then show the vertical shift as in [link] .

Got questions? Get instant answers now!

Got questions? Get instant answers now!

Try It

Graph $f(x)=4+2x,$ using transformations.

Got questions? Get instant answers now!

Writing the equation for a function from the graph of a line

Recall that in Linear Functions , we wrote the equation for a linear function from a graph. Now we can extend what we know about graphing linear functions to analyze graphs a little more closely. Begin by taking a look at [link] . We can see right away that the graph crosses the y -axis at the point $\left(0,\text{ 4}\right)$ so this is the y -intercept.