PSAT Math : How to find domain and range of the inverse of a relation

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 :How To Find Domain And Range Of The Inverse Of A Relation

What is the range of the functiony=x2+ 2?

Possible Answers:

{–2, 2}

all real numbers

undefined

{2}

y≥ 2

Correct answer:

y≥ 2

Explanation:

The range of a function is the set ofy-values that a function can take. First let's find the domain. The domain is the set ofx-values that the function can take. Here the domain is all real numbers because nox-value will make this function undefined. (Dividing by 0 is an example of an operation that would make the function undefined.)

So if any value ofxcan be plugged intoy=x2+ 2, canytake any value also? Not quite! The smallest value thatycan ever be is 2. No matter what value ofxis plugged in,y=x2+ 2 will never produce a number less than 2. Therefore the range isy≥ 2.

Example Question #1 :How To Find Domain And Range Of The Inverse Of A Relation

Which of the following values ofxis not in the domain of the functiony= (2x –1) / (x2– 6x+ 9) ?

Possible Answers:

1/2

0

2

–1/2

3

Correct answer:

3

Explanation:

Values ofxthat make the denominator equal zero are not included in the domain. The denominator can be simplified to (x –3)2, so the value that makes it zero is 3.

Example Question #2 :How To Find Domain And Range Of The Inverse Of A Relation

Given the relation below:

{(1, 2), (3, 4), (5, 6), (7, 8)}

Find the range of the inverse of the relation.

Possible Answers:
the inverse of the relation does not exist
{5, 6, 7, 8}
{2, 4, 6, 8}
{1, 2, 3, 4}
{1, 3, 5, 7}
Correct answer:{1, 3, 5, 7}
Explanation:

The domain of a relation is the same as the range of the inverse of the relation. In other words, the x-values of the relation are the y-values of the inverse.

Example Question #3 :How To Find Domain And Range Of The Inverse Of A Relation

What is the range of the functiony=x2+ 2?

Possible Answers:

undefined

all real numbers

{–2, 2}

y≥ 2

{2}

Correct answer:

y≥ 2

Explanation:

The range of a function is the set ofy-values that a function can take. First let's find the domain. The domain is the set ofx-values that the function can take. Here the domain is all real numbers because nox-value will make this function undefined. (Dividing by 0 is an example of an operation that would make the function undefined.)

So if any value ofxcan be plugged intoy=x2+ 2, canytake any value also? Not quite! The smallest value thatycan ever be is 2. No matter what value ofxis plugged in,y=x2+ 2 will never produce a number less than 2. Therefore the range isy≥ 2.

Example Question #1 :How To Find Domain And Range Of The Inverse Of A Relation

What is the smallest value that belongs to the range of the functionf(x)=2|4-x|-2?

Possible Answers:

\dpi{100} -2

\dpi{100} -4

\dpi{100} 4

\dpi{100} 0

\dpi{100} 2

Correct answer:

\dpi{100} -2

Explanation:

We need to be careful here not to confuse the domain and range of a function. The problem specifically concerns the range of the function, which is the set of possible numbers of\dpi{100} f(x). It can be helpful to think of the range as all the possible y-values we could have on the points on the graph of\dpi{100} f(x).

Notice that\dpi{100} f(x)has\dpi{100} |4-x|in its equation. Whenever we have an absolute value of some quantity, the result will always be equal to or greater than zero. In other words, |4-x|\geq0. We are asked to find the smallest value in the range of\dpi{100} f(x), so let's consider the smallest value of\dpi{100} |4-x|, which would have to be zero. Let's see what would happen to\dpi{100} f(x)if\dpi{100} |4-x|=0.

\dpi{100} f(x)=2(0)-2=0-2=-2

This means that when\dpi{100} |4-x|=0,\dpi{100} f(x)=-2. Let's see what happens when\dpi{100} |4-x|gets larger. For example, let's let\dpi{100} |4-x|=3.

\dpi{100} f(x)=2(3)-2=4

As we can see, as\dpi{100} |4-x|gets larger, so does\dpi{100} f(x). We want\dpi{100} f(x)to be as small as possible, so we are going to want\dpi{100} |4-x|to be equal to zero. And, as we already determiend,\dpi{100} f(x)equals\dpi{100} -2when\dpi{100} |4-x|=0.

The answer is\dpi{100} -2.

Example Question #5 :How To Find Domain And Range Of The Inverse Of A Relation

Iff(x) = x - 3, then findf^{-1}(x)

Possible Answers:

f^{-1}(x)=3-x

f^{-1}(x)=x+3

f^{-1}(x)=x-3

f^{-1}(x)=\frac{1}{x-3}

f^{-1}(x)=3x

Correct answer:

f^{-1}(x)=x+3

Explanation:

f(x) = x - 3is the same asy= x - 3.

To find the inverse simply exchangexandyand solve fory.

So we getx=y-3which leads toy=x+3.

Example Question #1 :How To Find Domain And Range Of The Inverse Of A Relation

If, then which of the following is equal to?

Possible Answers:

Correct answer:

Explanation:

Inversef2

Inverse3

Example Question #2 :How To Find Domain And Range Of The Inverse Of A Relation

Given the relation below, identify the domain of the inverse of the relation.

Possible Answers:

The inverse of the relation does not exist.

Correct answer:

Explanation:

The domain of the inverse of a relation is the same as the range of the original relation. In other words, the y-values of the relation are the x-values of the inverse.

For the original relation, the range is:.

Thus, the domain for the inverse relation will also be.

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