Find the Domain of Rational Functions

A step by step tutorial, with detailed solutions, on how to find the domain of rational functions is presented. Problems matched to the exercises with solutions at the bottom of the page are also presented.

Definition of the Domain of a Function

For a function f defined by an expression with variable x, the implied domain of f is the set of all real numbers variable x can take such that the expression defining the function is real. The domain can also be given explicitly.

also Step by Step Calculator to Find Domain of a Function




Example 1: Find the domain of function f defined by

rational function example 1

Solution to Example 1

  • f(x) can take real values if the denominator of f(x) is NOT ZERO because division by zero is not allowed in mathematics

    x - 2 ≠ 0

  • Solve the above inequality for to obtain the domain: x ≠ 2

  • Which in interval form may be written as follows

    (-∞ , 2) ∪ (2 , +∞)

Matched Problem 1: Find the domain of function f defined by

rational function  - matched problem 1




Example 2: Find the domain of function f defined by

rational function example 2

Solution to Example 2

  • For f(x) to have real values, the denominator must be different from zero. Hence

    x2 + 7 ≠ 0
  • Expression x2 + 7 is always positive (square added to a positive number). Hence the domain of f is given by the interval

    (-∞ , +∞)

Matched Problem 2: Find the domain of function f defined by:

rational function matched 2



Example 3: Find the domain of function f defined by:

rational function example 3

Solution to Example 3

  • For f(x) given above to be real, its denominator must be different from zero. Let us first find the roots of the denominator by solving the equation
    2x 2 + x - 15 = 0

  • The roots are

    - 3 and 5 / 2

  • The denominator 2x 2 + x - 15 is not equal to zeros for all real values except - 3 and 5 / 2. Hence the domain of the given function is given by

    (-∞ , -3) ∪ (-3 , 5 / 2) ∪ (5 / 2 , +∞)

Matched Problem 3: Find the domain of function f defined by:

rational function matched 3



Example 4: Find the domain of function f defined by:

rational function example 4

Solution to Example 4

  • For f(x) to be real, both denominators:2x - 6 and - 4x + 7 must not be equal to zero. Let us find the values of x that make the two denominators equal to zero
    2x - 6 = 0 gives x = 3

    4x + 7 gives x = - 7 / 4

  • f(x) is real for all real values except 3 and - 7/4. The domain of the above function is given by
    (-∞ , -7/4) ∪ (-7/4 , 3) ∪ (3 , +∞)

Matched Problem 4: Find the domain of function f defined by:

rational function matched 4



Example 5: Find the domain of function f defined by:

rational function example 5

Solution to Example 5

  • Let us first find the values that makes the denominator equal to zero
    x3 - x = 0

  • Factor the expression on the left hand side of the equation
    x(x2 - 1) = 0

    x(x + 1)(x - 1) = 0

  • Solve the above equation to obtain the solution set as follows:
    x = 0, x = 1 and x = -1

  • The domain is given in interval form as follows:
    (-∞ , -1) ∪ (-1 , 0) ∪ (0 , 1) ∪ (1 , +∞)

Matched Problem 5: Find the domain of function f defined by:

rational function matched 5



Answers to matched problems
  1. (-∞ , 6) ∪ (6 , + ∞)
  2. (-∞ , + ∞)
  3. (-∞ , - 4 / 3) ∪ (- 4 / 3 , 2) ∪ (2 , ∞)
  4. (-∞ , - 4) ∪ (- 4 , 3 / 7) ∪ (3 / 7 , ∞)
  5. (-∞ , -3) ∪ (-3 , 0) ∪ (0 , 1) ∪ (1 , ∞)



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