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Precalculus : Determine if a Relation is a Function

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Example Questions

Example Question #1 : Determine If A Relation Is A Function

Which of the following expressions is not a function?

Possible Answers:

y=sinx

$y=\pm\sqrt{x^2-7x+5}$

y=2^x+343

y=5x+99

y=6x^2-54x+76

Correct answer:

$y=\pm\sqrt{x^2-7x+5}$

Explanation:

Recall that an expression is only a function if it passes the vertical line test. Test this by graphing each function and looking for one which fails the vertical line test. (The vertical line test consists of drawing a vertical line through the graph of an expression. If the vertical line crosses the graph of the expression more than once, the expression is not a function.)

Functions can only have one y value for every x value. The only choice that reflects this is:

$y=\pm\sqrt{x^2-7x+5}$

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Example Question #1 : Determine If A Relation Is A Function

Suppose we have the relation $\small (a,b)$ on the set of real numbers $\mathbb{R}$ whenever $\small a<b$ . Which of the following is true.

Possible Answers:

The relation is a function because every relation is a function, since that's how relations are defined.

The relation is a function because $\small (2,4)$ holds and $\small (3,4)$ also holds.

The relation is not a function because $\small (2,3)$ and $\small (2,4)$ both hold.

The relation is a function because for every $\small a$ , there is only one $\small b$ such that $\small (a,b)$ holds.

The relation is not a function because $\small (0,1)$ holds but $\small (1,0)$ does not.

Correct answer:

The relation is not a function because $\small (2,3)$ and $\small (2,4)$ both hold.

Explanation:

The relation is not a function because $\small (2,3)$ and $\small (2,4)$ hold. If it were a function, $\small (2,b)$ would hold only for one $\small b$ . But we know it holds for $\small b=3,4$ because $\small 2<3$ and $\small 2<4$ . Thus, the relation $\small (a,b)$ on the set of real numbers $\small \mathbb{R}$ is not a function.

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Example Question #1 : Determine If A Relation Is A Function

Consider a family consisting of a two parents, Juan and Oksana, and their daughters Adriana and Laksmi. A relation $\small (a,b)$ is true whenever $\small b$ is the child of $\small a$ . Which of the following is not true?

Possible Answers:

(Adriana,Laksmi) does not hold because Laksmi is not Adriana's child and is a boy.

The relation is not a function because (Laksmi,Juan) and (Laksmi,Oksana) both hold.

Even if the two parents had only one daughter, the relation would not be a function.

If the two parents had only one daughter, the relation would be a function.

(Laksmi,Adriana) does not hold because Adriana is not Laksmi's child.

Correct answer:

Even if the two parents had only one daughter, the relation would not be a function.

Explanation:

The statement

"Even if the two parents had only one daughter, the relation would not be a function."

is not true because if they had only one daughter, say Adriana, then the only relations that would exist would be (Juan, Adriana) and (Oksana,Adriana), which defines a function.

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Example Question #1 : Determine If A Relation Is A Function

Which of the following relations is not a function?

Possible Answers:

$y = x^{\frac{3}{2}}$

y=2

x = y^2

$y = \sqrt{x}$

Correct answer:

x = y^2

Explanation:

The definition of a function requires that for each input (i.e. each value of ), there is only one output (i.e. one value of ). For x = y^2 , each value of corresponds to two values of (for example, when x = 4 , both y=2 and y = -2 are correct solutions). Therefore, this relation cannot be a function.

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Example Question #2 : Determine If A Relation Is A Function

Given the set of ordered pairs, determine if the relation is a function

(0,2)

(1,4)

(2,6)

(3,8)

(2,5)

Possible Answers:

Cannot be determined

Yes

Correct answer:

Explanation:

A relation is a function if no single x-value corresponds to more than one y-value.

Because the mapping from goes to and

the relation is NOT a function.

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Example Question #1 : Determine If A Relation Is A Function

What equation is perpendicular to $y = \frac{5}{2}x+12$ and passes throgh (2,5) ?

Possible Answers:

$y = -\frac{2}{5}x + \frac{29}{5}$

$y = \frac{2}{5}x + \frac{21}{5}$

$y = -\frac{5}{2}x + \frac{22}{5}$

$y = -\frac{2}{5}x + \frac{27}{5}$

$y = -\frac{4}{5}x + \frac{26}{5}$

Correct answer:

$y = -\frac{2}{5}x + \frac{29}{5}$

Explanation:

First find the reciprocal of the slope of the given function.
$m_2 = -\frac{1}{m_1}$

$m_2 = -\frac{2}{5}$

The perpendicular function is:

$y = -\frac{2}{5}x+c$

Now we must find the constant, , by using the given point that the perpendicular crosses.

$5 = -\frac{2}{5}(2)+c$

solve for :

$c = \frac{29}{5}$

$y = -\frac{2}{5}x + \frac{29}{5}$

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Example Question #2 : Determine If A Relation Is A Function

Is the following relation of ordered pairs a function?

(1,1); (2,2); (3,1); (4,2); (5,1); (6,7)

Possible Answers:

Cannot be determined

Yes

Correct answer:

Yes

Explanation:

A set of ordered pairs is a function if it passes the vertical line test.

Because there are no more than one corresponding value for any given value, the relation of ordered pairs IS a function.

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Precalculus : Determine if a Relation is a Function

Study concepts, example questions & explanations for Precalculus

All Precalculus Resources

Example Questions

Example Question #1 : Determine If A Relation Is A Function

Example Question #1 : Determine If A Relation Is A Function

Example Question #1 : Determine If A Relation Is A Function

Example Question #1 : Determine If A Relation Is A Function

Example Question #2 : Determine If A Relation Is A Function

Example Question #1 : Determine If A Relation Is A Function

Example Question #2 : Determine If A Relation Is A Function

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