GMAT Math : DSQ: Understanding functions

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #71 : Algebra

A relation comprises ten ordered pairs. Is it a function?

Statement 1: Its domain is .

Statement 2: The line  passes through its graph twice.

Possible Answers:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

If Statement 1 alone is assumed, then, since there are only six domain elements and ten points in the relation, at least one of the domain elements must match with more than one range element. This forces the relation to not be a function.

If Statement 2 alone is assumed, then, since  is a vertical line that passes through the graph twice, the relation fails the vertical line test and is therefore not a function.

Example Question #42 : Dsq: Understanding Functions

Define  and  to be functions. Does  have an inverse?

Statement 1:  has an inverse.

Statement 2:  has an inverse.

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

. This defintion will come into play here.

A function  has an inverse if and only if it is "one-to-one" - that is, if 

 if and only if .

 

Assume statement 1 only. 

 if and only if . However, since it is not known whether  has an inverse, it is possible for  with either  or . Transitively, it is possible for  with either  or .

 

Assume Statement 2 only.

  if and only if . But it is possible for  with  or  - and, subsequently, with  or . Transitively, it is possible for  with either  or .

 

Assume both statements are true. Then 

 if and only if , and  if and only if . Transitively,

 if and only if 

Therefore,

 if and only if , and, subsequently,   has an inverse.

 

The two statements together - but neither alone - lead to an answer.

Example Question #71 : Algebra

Define  and  to be functions on the real numbers. Does  have an inverse?

Statement 1:  does not have an inverse.

Statement 2:  does not have an inverse.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

Examine these two examples.

Example 1: Let 

Neither function has an inverse, since both functions pair all  values with the same  value, namely, 1. 

 does not have an inverse, since it pairs all  values with the same  value, namely, 0.

Therefore,  and  have no inverse, and  has no inverse.

 

Example 2: Let  and .

 has no inverse, since  and  - that is,  pairs at least two  values with the same  value.

 has no inverse, since  and ; that is,  pairs at least two  values with the same  value.

 is the identity function, which has itself as an inverse.

 

This demonstrates that, if  and  do not have inverses, it is possible for  to have an inverse or to not have an inverse. Therefore, the two statements together are inconclusive.

Example Question #76 : Algebra

Define  and  to be functions on the set of real numbers. Does  have an inverse?

Statement 1:  has an inverse.

Statement 2:  has an inverse.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

Examine these two examples.

 

Example 1:  and .

 can be proved to have an inverse as follows:

Switch the variables:

Therefore,  has an inverse; a smiliar proof shows that  has an inverse .

, which can be similarly be shown to have inverse 

.

 

Example 2:  and .

Again,  and  can be shown to have inverses,  and .

 

. However, 

.

This function has no inverse, since this function pairs multiple values of  with the same value of , 0.

 

In both cases, both  and  have inverses, but in one case,  has an inverse, and in the other case,  does not. The two statements together are inconclusive. 

Example Question #41 : Functions/Series

Is a given relation a function?

Statement 1: The line  passes through its graph infinitely many times.

Statement 2: The line  passes through its graph infinitely many times.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

If Statement 1 alone is assumed, then, since  is a vertical line that passes through the graph of the relation more than once, the relation fails the vertical line test, and the relation can be proved to not be a function.

However, there is no restriction on how many times a horizontal line such as  can pass through the graph of a relation for it to be or not to be a function. Statement 2 proves nothing either way.

Example Question #71 : Algebra

Is a given relation a function?

Statement 1: Two of its ordered pairs have -coordinate .

Statement 2: Two of its ordered pairs have -coordinate .

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

Statement 1 alone disproves that the relation is a function, since in a function, no -coordinate can be paired with two -coordinates. However, statement 2 alone does not prove or disprove the relation to be a function, since it is permissible for two -coordinates to be paired with the same -coordinate.

Example Question #41 : Dsq: Understanding Functions

The volume of a right circular cylinder is ; the radius of its base is .

Give the height of the cylinder.

Possible Answers:

Correct answer:

Explanation:

The volume of a right circular cylinder, given base with radius  and given height , is

.

Setting  and :

Example Question #46 : Dsq: Understanding Functions

The volume of a right circular cylinder is ; its height is .

Give the radius of a base of the cylinder.

Possible Answers:

Correct answer:

Explanation:

The volume of a right circular cylinder, given base with radius  and given height , is

.

Setting  and :

Example Question #41 : Functions/Series

The first two terms of a sequence,  and , are given specific values, which you are not given. Each successive term is equal to the sum of the two preceding terms. For example, the fifth term is equal to the sum of the third and fourth terms.

Calculate .

Statement 1: 

Statement 2: 

Possible Answers:

BOTH STATEMENTS TOGETHER provide sufficient information to answer the question, but NEITHER STATEMENT ALONE provides sufficient information to answer the question.

STATEMENT 2 ALONE provides sufficient information to answer the question, but STATEMENT 1 ALONE does NOT provide sufficient information to answer the question.

STATEMENT 1 ALONE provides sufficient information to answer the question, but STATEMENT 2 ALONE does NOT provide sufficient information to answer the question.

BOTH STATEMENTS TOGETHER do NOT provide sufficient information to answer the question.

EITHER STATEMENT ALONE provides sufficient information to answer the question.

Correct answer:

BOTH STATEMENTS TOGETHER provide sufficient information to answer the question, but NEITHER STATEMENT ALONE provides sufficient information to answer the question.

Explanation:

Suppose the first and second terms of the sequence are  and , respectively. The first eight terms are

, or 

, or  - this is the fifth term.

Continuing to add:

 - the eighth term.

From Statement 1 alone, we get that

From Statement 2 alone, we get that 

Both statements are equations in two variables, so neither statement alone tells us the value of either. But if we know both statements together, we have a system of equations that can be solved as follows:

 

                     

 

 - this is .

Example Question #82 : Algebra

The first two terms of a sequence,  and , are given specific values, which you are not given. Each successive term is equal to the sum of the two preceding terms. For example, the fifth term is equal to the sum of the third and fourth terms.

Evaluate .

Statement 1:  and 

Statement 2: The arithmetic mean of  and  is 99.5.

Possible Answers:

BOTH STATEMENTS TOGETHER provide sufficient information to answer the question, but NEITHER STATEMENT ALONE provides sufficient information to answer the question.

STATEMENT 2 ALONE provides sufficient information to answer the question, but STATEMENT 1 ALONE does NOT provide sufficient information to answer the question.

BOTH STATEMENTS TOGETHER do NOT provide sufficient information to answer the question.

STATEMENT 1 ALONE provides sufficient information to answer the question, but STATEMENT 2 ALONE does NOT provide sufficient information to answer the question.

EITHER STATEMENT ALONE provides sufficient information to answer the question.

Correct answer:

EITHER STATEMENT ALONE provides sufficient information to answer the question.

Explanation:

Assume Statement 1 alone. By the rule, , so by substitution:

.

Assume Statement 2 alone. The arithmetic mean of two numbers is half their sum, so

By the rule, , so 

.

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