Compressions and Stretches

Learning Objectives

  • Graph Functions Using Compressions and Stretches

Adding a constant to the inputs or outputs of a function changed the position of a graph with respect to the axes, but it did not affect the shape of a graph. We now explore the effects of multiplying the inputs or outputs by some quantity.

We can transform the inside (input values) of a function or we can transform the outside (output values) of a function. Each change has a specific effect that can be seen graphically.

Vertical Stretches and Compressions

When we multiply a function by a positive constant, we get a function whose graph is stretched or compressed vertically in relation to the graph of the original function. If the constant is greater than 1, we get a vertical stretch; if the constant is between 0 and 1, we get a vertical compression. The graph below shows a function multiplied by constant factors 2 and 0.5 and the resulting vertical stretch and compression.

Graph of a function that shows vertical stretching and compression.

Vertical stretch and compression

A General Note: Vertical Stretches and Compressions

Given a function [latex]f\left(x\right)[/latex], a new function [latex]g\left(x\right)=af\left(x\right)[/latex], where [latex]a[/latex] is a constant, is a vertical stretch or vertical compression of the function [latex]f\left(x\right)[/latex].

  • If [latex]a>1[/latex], then the graph will be stretched.
  • If 0 < a < 1, then the graph will be compressed.
  • If [latex]a<0[/latex], then there will be combination of a vertical stretch or compression with a vertical reflection.

How To: Given a function, graph its vertical stretch.

  1. Identify the value of [latex]a[/latex].
  2. Multiply all range values by [latex]a[/latex].
  3. If [latex]a>1[/latex], the graph is stretched by a factor of [latex]a[/latex].
    If [latex]{ 0 }<{ a }<{ 1 }[/latex], the graph is compressed by a factor of [latex]a[/latex].
    If [latex]a<0[/latex], the graph is either stretched or compressed and also reflected about the x-axis.

Example: Graphing a Vertical Stretch

A function [latex]P\left(t\right)[/latex] models the number of fruit flies in a population over time, and is graphed below.

A scientist is comparing this population to another population, [latex]Q[/latex], whose growth follows the same pattern, but is twice as large. Sketch a graph of this population.

Graph to represent the growth of the population of fruit flies.

Try It

How To: Given a tabular function and assuming that the transformation is a vertical stretch or compression, create a table for a vertical compression.

  1. Determine the value of [latex]a[/latex].
  2. Multiply all of the output values by [latex]a[/latex].

Example: Finding a Vertical Compression of a Tabular Function

A function [latex]f[/latex] is given in the table below. Create a table for the function [latex]g\left(x\right)=\frac{1}{2}f\left(x\right)[/latex].

[latex]x[/latex] 2 4 6 8
[latex]f\left(x\right)[/latex] 1 3 7 11

Try It

A function [latex]f[/latex] is given below. Create a table for the function [latex]g\left(x\right)=\frac{3}{4}f\left(x\right)[/latex].

[latex]x[/latex] 2 4 6 8
[latex]f\left(x\right)[/latex] 12 16 20 0

Example: Recognizing a Vertical Stretch

Graph of a transformation of f(x)=x^3.

The graph is a transformation of the toolkit function [latex]f\left(x\right)={x}^{3}[/latex]. Relate this new function [latex]g\left(x\right)[/latex] to [latex]f\left(x\right)[/latex], and then find a formula for [latex]g\left(x\right)[/latex].

Try It 6

Write the formula for the function that we get when we stretch the identity toolkit function by a factor of 3, and then shift it down by 2 units.
Check your work with Desmos.

Horizontal Stretches and Compressions

Graph of the vertical stretch and compression of x^2.

Now we consider changes to the inside of a function. When we multiply a function’s input by a positive constant, we get a function whose graph is stretched or compressed horizontally in relation to the graph of the original function. If the constant is between 0 and 1, we get a horizontal stretch; if the constant is greater than 1, we get a horizontal compression of the function.

Given a function [latex]y=f\left(x\right)[/latex], the form [latex]y=f\left(bx\right)[/latex] results in a horizontal stretch or compression. Consider the function [latex]y={x}^{2}[/latex]. The graph of [latex]y={\left(0.5x\right)}^{2}[/latex] is a horizontal stretch of the graph of the function [latex]y={x}^{2}[/latex] by a factor of 2. The graph of [latex]y={\left(2x\right)}^{2}[/latex] is a horizontal compression of the graph of the function [latex]y={x}^{2}[/latex] by a factor of 2.

A General Note: Horizontal Stretches and Compressions

Given a function [latex]f\left(x\right)[/latex], a new function [latex]g\left(x\right)=f\left(bx\right)[/latex], where [latex]b[/latex] is a constant, is a horizontal stretch or horizontal compression of the function [latex]f\left(x\right)[/latex].

  • If [latex]b>1[/latex], then the graph will be compressed by [latex]\frac{1}{b}[/latex].
  • If [latex]0<b<1[/latex], then the graph will be stretched by [latex]\frac{1}{b}[/latex].
  • If [latex]b<0[/latex], then there will be combination of a horizontal stretch or compression with a horizontal reflection.

How To: Given a description of a function, sketch a horizontal compression or stretch.

  1. Write a formula to represent the function.
  2. Set [latex]g\left(x\right)=f\left(bx\right)[/latex] where [latex]b>1[/latex] for a compression or [latex]0<b<1[/latex]
    for a stretch.

Example: Graphing a Horizontal Compression

Suppose a scientist is comparing a population of fruit flies to a population that progresses through its lifespan twice as fast as the original population. In other words, this new population, [latex]R[/latex], will progress in 1 hour the same amount as the original population does in 2 hours, and in 2 hours, it will progress as much as the original population does in 4 hours. Sketch a graph of this population.

Example: Finding a Horizontal Stretch for a Tabular Function

A function [latex]f\left(x\right)[/latex] is given below. Create a table for the function [latex]g\left(x\right)=f\left(\frac{1}{2}x\right)[/latex].

[latex]x[/latex] 2 4 6 8
[latex]f\left(x\right)[/latex] 1 3 7 11

Example: Recognizing a Horizontal Compression on a Graph

Relate the function [latex]g\left(x\right)[/latex] to [latex]f\left(x\right)[/latex].

Graph of f(x) being vertically compressed to g(x).

Try It

Write a formula for the toolkit square root function horizontally stretched by a factor of 3.
Use Desmos to check your work.