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Graphs of Square Root Functions

Graph root functions with and without a calculator

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Graphs of Square Root Functions

Graphs of Square Root Functions

In this chapter you’ll learn about a different kind of function called the square root function. You’ve seen that taking the square root is very useful in solving quadratic equations. For example, to solve the equation x2=25 we take the square root of both sides: x2=±25, so x=±5.

A square root function is any function with the form: y=af(x)+c —in other words, any function where an expression in terms of x is found inside a square root sign (also called a “radical” sign), although other terms may be included as well.

 

Graph and Compare Square Root Functions

When working with square root functions, you’ll have to consider the domain of the function before graphing. The domain is very important because the function is undefined when the expression inside the square root sign is negative, and as a result there will be no graph in whatever region of xvalues makes that true.

To discover how the graphs of square root functions behave, let’s make a table of values and plot the points.

Graphing Functions

Graph the function y=x.

Before we make a table of values, we need to find the domain of this square root function. The domain is found by realizing that the function is only defined when the expression inside the square root is greater than or equal to zero. Since the expression inside the square root is just x, that means the domain is all values of x such that x0.

This means that when we make our table of values, we should pick values of x that are greater than or equal to zero. It is very useful to include zero itself as the first value in the table and also include many values greater than zero. This will help us in determining what the shape of the curve will be.

x y=x
0 y=0=0
1 y=1=1
2 y=2=1.4
3 y=3=1.7
4 y=4=2
5 y=5=2.2
6 y=6=2.4
7 y=7=2.6
8 y=8=2.8
9 y=9=3

Here is what the graph of this table looks like:

License: CC BY-NC 3.0

The graphs of square root functions are always curved. The curve above looks like half of a parabola lying on its side, and in fact it is. It’s half of the parabola that you would get if you graphed the expression y2=x. And the graph of y=x is the other half of that parabola:

License: CC BY-NC 3.0

Notice that if we graph the two separate functions on the same coordinate axes, the combined graph is a parabola lying on its side.

License: CC BY-NC 3.0

Now let's compare square root functions that are multiples of each other.

Graphing Multiple Functions on the Same Graph 

1. Graph the functions y=x,y=2x,y=3x, and y=4x on the same graph.

Here is just the graph without the table of values:

License: CC BY-NC 3.0

If we multiply the function by a constant bigger than one, the function increases faster the greater the constant is.

2. Graph the functions y=x,y=2x,y=3x, and y=4x on the same graph.

License: CC BY-NC 3.0

Notice that multiplying the expression inside the square root by a constant has the same effect as multiplying by a constant outside the square root; the function just increases at a slower rate because the entire function is effectively multiplied by the square root of the constant.

Also note that the graph of 4x is the same as the graph of 2x. This makes sense algebraically since 4=2.

3. Graph the functions y=x,y=12x,y=13x, and y=14x on the same graph.

License: CC BY-NC 3.0

If we multiply the function by a constant between 0 and 1, the function increases more slowly the smaller the constant is.

 

Examples

Graph the functions

Example 1

y=2x and y=2x on the same graph. 

If we multiply the whole function by -1, the graph is reflected about the xaxis.

License: CC BY-NC 3.0

Example 2

y=x and y=x on the same graph.

License: CC BY-NC 3.0

On the other hand, when just the x is multiplied by -1, the graph is reflected about the yaxis. Notice that the function y=x has only negative xvalues in its domain, because when x is negative, the expression under the radical sign is positive.

Review

Graph the following functions.

  1. y=3x
  2. y=12x
  3. y=4x
  4. y=x+7
  5. y=2x5
  6. y=3x+12

Graph the following functions on the same coordinate axes.

  1. \begin{align*}y = \sqrt{x}, y = 2.5\sqrt{x}\end{align*} and \begin{align*} y= -2.5\sqrt{x}\end{align*}
  2. \begin{align*}y = \sqrt{x}, y = 0.3 \sqrt{x}\end{align*} and \begin{align*} y= 0.6\sqrt{x}\end{align*}
  3. \begin{align*}y = \sqrt{x}, y = \sqrt{x - 5}\end{align*} and \begin{align*} y= \sqrt{x + 5}\end{align*}
  4. \begin{align*}y = \sqrt{x}, y = \sqrt{x} + 8\end{align*} and \begin{align*} y= \sqrt{x} - 8\end{align*}

Review (Answers)

To view the Review answers, open this PDF file and look for section 11.1. 

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Vocabulary

square root function

A square root function is a function with the parent function y=\sqrt{x}.

Image Attributions

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