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Reciprocal Graphs

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Reciprocal Graphs

Reciprocal Graphs are graphical representations of reciprocal functions generically represented as and , where the numerator is a real constant, and the denominator contains an algebraic expression with a variable x.

Using reciprocal graphs

Reciprocal graphs are useful to visually represent relationships that are inversely proportional, which means that they behave in opposite ways – if one decreases, the other one increases, and vice versa. For example, if the number of workers in a shop increases, the amount of time that the customers spend waiting to be served will be reduced.

Asymptotes

To sketch this type of graph, you need to take into account its asymptotes. An asymptote is a line that the curve gets very close to, but never touches. The graph of reciprocal functions and have asymptotes at and .

See the graph below for

Reciprocal Graphs Reciprocal function asymptotes StudySmarterReciprocal function asymptotes, Marilú García De Taylor - StudySmarter Originals

is a vertical asymptote because you cannot divide by zero; therefore, x cannot be zero. is a horizontal asymptote because there are no values of x that make , so y cannot be zero either.

Notice that the graph of is symmetric to the lines and .

Reciprocal Graphs Reciprocal function symmetry StudySmarterReciprocal function y = 1 / x - symmetry to y = x, Marilú García De Taylor - StudySmarter Originals

Reciprocal Graphs Reciprocal function symmetry StudySmarterReciprocal function y = 1 / x - symmetry to y = -x, Marilú García De Taylor - StudySmarter Originals

In general, the domain of reciprocal functions will be all real numbers apart from the vertical asymptote, and the range will be all real numbers apart from the horizontal asymptote.

Types of reciprocal graphs

The Graphs article discusses that the coordinate plane is divided into four quadrants named using roman numbers (I, II, III and IV):

Reciprocal Graphs Coordinate plane StudySmarterCoordinate plane, Marilú García De Taylor - StudySmarter Originals

The possible types of reciprocal graphs include:

a) If a> 0:

For example, if , , the shape of the graph is shown below. Notice that the graph is drawn on quadrants I and III of the coordinate plane.

Reciprocal Graphs Reciprocal function positive numerator StudySmarterReciprocal function, Marilú García De Taylor - StudySmarter Originals

b) If a <0:

For example, if , , the shape of the reciprocal function is shown below. In this case, the graph is drawn on quadrants II and IV. This graph is the reflection of the previous one because the negative sign in the function means that all positive values of will now have negative values of y, and all negative values of x will now have positive values of y.

Reciprocal Graphs Reciprocal function negative numerator StudySmarterReciprocal function with negative numerator, Marilú García De Taylor - StudySmarter Originals

a) If a> 0:

For example, if , , the shape of the graph is shown below. Notice that the graph is drawn on quadrants I and II of the coordinate plane. The shape of the graph of changes in comparison to the previous graph of , because having in the denominator means that all values of y will be positive for all values of .

Reciprocal Graphs Reciprocal squared function StudySmarterReciprocal squared function, Marilú García De Taylor - StudySmarter Originals

b) If a <0:

For example, if , , the shape of the reciprocal function is shown below. In this case, the graph is drawn on quadrants III and IV. This graph is also the reflection of the previous one due to the negative sign in the numerator of the function.

Reciprocal Graphs Reciprocal squared function negative numerator StudySmarterReciprocal squared function with negative numerator, Marilú García De Taylor - StudySmarter Originals

Drawing reciprocal graphs

To show you how to draw the graph of a reciprocal function, we will use the example of . To graph this function you need to follow these steps:

  • Identify the vertical and horizontal asymptotes.

For , and are asymptotes.

  • Identify the type of reciprocal function or , and if a is positive or negative. This information will give you an idea of where the graphs will be drawn on the coordinate plane. This step is optional.

In our example , the reciprocal function is of type y = and a> 0; therefore, the graphs will be drawn on quadrants I and III.

  • Plot points strategically reveal the graph's behaviour as it approaches the asymptotes from each side.

Negative side:

Notice that the further we go to the left, the closer we get to zero.

Now let's try some fractions of negative 1:

Positive side:

Notice that the further we go to the right, the closer we get to zero.

Now let's try some fractions of positive 1:

x
y
  • Draw the graph using the table of values obtained:

Reciprocal Graphs Reciprocal function graph StudySmarterReciprocal function graph, Marilú García De Taylor - StudySmarter Originals

A reciprocal function has been transformed if its equation is written in the standard form , where a, h and k are real constants, the vertical asymptote of the function is , and the horizontal one is .

For the reciprocal function , the asymptotes are and .

Reciprocal Graphs Reciprocal graph equation standard form StudySmarterReciprocal graph with the equation in standard form, Marilú García De Taylor - StudySmarter Originals

You might be asked to find the interceptions of the reciprocal function graph with the x and y axes. You can proceed as follows:

  • x-intercept: Substitute y = 0 in the equation and solve for x.

The point where the graph of the function crosses the x-axis is (-3, 0)

  • y-intercept: Substitute x = 0 in the equation and solve for y.

The point where the graph of the function crosses the y-axis is

How do you find the equation of a reciprocal graph?

If you are given a reciprocal graph, you can find its equation by following these steps:

  1. Find the vertical asymptote. This is the value you need to add or subtract from the variable in the denominator . It will have the opposite sign of the vertical asymptote.

  2. Find the horizontal asymptote. This will be the value of , which is added or subtracted from the fraction depending on its sign.

  3. Find the value of by substituting the x and y corresponding to a given point on the curve in the equation.

Find the equation for the reciprocal graph below:

Reciprocal Graphs Equation of a reciprocal graph StudySmarterEquation of a reciprocal graph, Marilú García De Taylor - StudySmarter Originals

  1. Vertical asymptote , therefore
  2. Horizontal asymptote , therefore
  3. Substitute the point A (-5, 0) in the reciprocal function to find the value of :

The equation of the reciprocal function is

Finding the reciprocal of a function

We know from Algebra that you can calculate the reciprocal of a number by swapping the numerator and the denominator. The same applies to functions. To find the reciprocal of a function you can find the expression .

Find the reciprocal of the function

The reciprocal of is

Reciprocal Graphs - Key takeaways

  • Reciprocal graphs are graphical representations of reciprocal functions, where the numerator is a real constant, and the denominator contains an algebraic expression with a variable x.
  • To sketch reciprocal graphs, you must take into account their asymptotes.
  • An asymptote is a line that the curve gets very close to, never touching it.
  • The domain of reciprocal functions will be all real numbers apart from the vertical asymptote.
  • The range of reciprocal functions will be all real numbers apart from the horizontal asymptote.

Frequently Asked Questions about Reciprocal Graphs

Reciprocal graphs are graphical representations of reciprocal functions, where the numerator is a real constant, and the denominator contains an algebraic expression with a variable x.

To graph this function you need to follow these steps:

  • Identify the vertical and horizontal asymptotes. 
  • Identify the type of reciprocal function y = a/x or y = a/x², and if a is positive or negative. This information will give you an idea of where the graphs will be drawn on the coordinate plane. (Optional)
  • Plot points strategically to reveal the behaviour of the graph as it approaches the asymptotes from each side.
  • Draw the graph using the table of values obtained.


To find the equation of a reciprocal function y = a/(x+h) + k follow these steps:

  1. Find the vertical asymptote. This is the value that you need to add or subtract from the variable in the denominator (h). h will have the opposite sign of the vertical asymptote. 
  2. Find the horizontal asymptote. This will be the value of k, which is added or subtracted from the fraction depending on its sign.
  3. Find the value of a by substituting the values of x and y corresponding to a given point on the curve in the equation.

To find the reciprocal of a function f(x) you can find the expression 1/f(x).

Reciprocal graphs are useful to visually represent relationships that are inversely proportional, which means that they behave in opposite ways. If one decreases the other one increases, and vice versa.

Final Reciprocal Graphs Quiz

Question

What are reciprocal graphs?

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Answer

Reciprocal graphs are graphical representations of reciprocal functions, where the numerator is a real constant, and the denominator contains an algebraic expression with a variable x.

Show question

Question

What is an asymptote?

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Answer

An asymptote is a line that the curve of a reciprocal graph gets very close to, but it never touches it.

Show question

Question

What is the domain of a reciprocal function?


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Answer

The domain of reciprocal functions will be all real numbers apart from the vertical asymptote.

Show question

Question

What is the range of a reciprocal function?


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Answer

The range of reciprocal functions will be all real numbers apart from the horizontal asymptote.

Show question

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