GMAT Math : DSQ: Understanding functions

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Example Question #1 : Dsq: Understanding Functions

Define f (x) = 4x - 9 .

Evaluate $(f \circ g) (9)$ .

Statement 1: g (9) = 8

Statement 2: g (12) = 9

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.

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.

Correct answer:

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

Explanation:

If you know that g (9) = 8 , then you can calculate:

$(f \circ g) (9) = f (\; g(9)\; ) = f(8) = 4\cdot8 -9 = 23$

Knowing the -values of that are paired with -value 9, however, is neither necessary nor useful here.

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Example Question #2 : Dsq: Understanding Functions

The first term of an arithmetic sequence is 100. What is the second term?

Statement 1: The sum of the third and fourth terms is 320.

Statement 2: The tenth term subtracted from the twelfth term yields a difference of 48.

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.

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

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

Let be the common difference of the sequence. Then the third and fourth terms are, respectively, 100 + 2d and 100+3d . If their sum is 320, then

(100+2d) + (100+3d) =320

200+5d = 320

5d = 120

d = 24

If the difference between the twelfth term 100+11d and tenth term 100+9d is 24, then

(100+11d) - (100+9d) = 48

2d= 48

d = 24

From either statement, the common difference can be calculated, then added to 100 to get the second term, 124.

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Example Question #1 : Dsq: Understanding Functions

What is the first term of a geometric sequence?

Statement 1: The product of the second and third terms is 4,096.

Statement 2: The product of the first and fourth terms is 4,096.

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.

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.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

Let be the first term and be the common ratio. Then the first four terms are:

$A,AR,AR^{2},AR^{3}$

The two statements are equivalent to the equations $AR \cdot AR^{2} = 4.096$ and $A \cdot AR^{3} = 4,096$ . But they turn out to be equivalent as can be seen here:

$AR \cdot AR^{2} = 4096$

$A \cdot A \cdot R \cdot R^{2} = 4096$

$A^{2}R^{3} = 4.096$

$A \cdot AR^{3} = 4,096$

$A^{2}R^{3} = 4,096$

This (common) statement alone is not enough to allow us to calculate .

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Example Question #2 : Dsq: Understanding Functions

This relation has five different ordered pairs: is it a function?

$\begin{matrix} \underline{x}&\underline{y}\\ 1& A\\ 2& A\\ 3& A\\ B&A + 1 \\ 5& A + 2 \end{matrix}$

Statement 1: A = 3

Statement 2: B = 5

Possible Answers:

EITHER 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.

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:

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

Explanation:

To prove that a relation is a function, you must prove that no -coordinate is matched with more than one -coordinate. Statement 2 proves that this is false, since 5 is now matched with both and A + 1 , which are different numbers regardless of . Statement 1 is irrelevant, since it does not prove or disprove this condition.

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Example Question #5 : Dsq: Understanding Functions

$f ^{-1} (A) = 40$

Evaluate .

Statement 1: The graph of includes the point (40, 19) .

Statement 2: f(19) = 40

Possible Answers:

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.

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

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 $f ^{-1} (A) = 40$ , then f(40) = A , so this question is equivalent to evaluating f(40) .

If the ordered pair (40, 19) is on the graph of , then f(40) = 19 , so A = 19 .

Knowing that f(19) = 40 is of no help, as this just tells us $f ^{-1} (40) = 19$ .

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Example Question #3 : Dsq: Understanding Functions

What is the first term of the geometric sequence?

Statement 1: The sum of the second and third terms is 90.

Statement 2: The sum of the third and fourth terms is 450.

Possible Answers:

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.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT 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 sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

Let be the first term and be the common ratio. Then the first four terms are:

$A,AR,AR^{2},AR^{3}$ .

The two statements below are equivalent to $AR+ AR^{2} = 90$ and $AR^{2}+AR^{3} = 450$ , respectively. Neither, alone, will help you figure out or . If you know both, you can use algebra to deduce their values:

$AR^{2}(1+R) = 450$

AR (1 + R) = 90

Divide both sides of the first equation by both sides of the second:

R = 5

Substitute this value into either equation. We'll use the 2nd equation:

$A \cdot 5 \cdot (1 + 5) = 90$

$A \cdot 30 = 90$

A = 3

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Example Question #6 : Dsq: Understanding Functions

$\left \lfloor x\right \rfloor$ is defined to be the greatest integer less than or equal to .

Evaluate $\left \lfloor \frac{A}{B} \right \rfloor$

Statement 1: A + 4 = B

Statement 2: B = 2A

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 insufficient 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.

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

Correct answer:

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

Explanation:

Statement 1 is not enough to answer this question.

Case 1: A = 1,B= 5

Then $\left \lfloor \frac{A}{B} \right \rfloor = \left \lfloor \frac{1}{5} \right \rfloor = 0$

Case 2: A = -1,B= 3

Then $\left \lfloor \frac{A}{B} \right \rfloor = \left \lfloor \frac{-1}{3} \right \rfloor = \left \lfloor - \frac{1}{3} \right \rfloor = -1$

If Statement 2 is true, however:

$\left \lfloor \frac{A}{B} \right \rfloor = \left \lfloor \frac{A}{2A} \right \rfloor = \left \lfloor \frac{1}{2} \right \rfloor =0$

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Example Question #7 : Dsq: Understanding Functions

This relation has the following five ordered pairs: is it a function?

$\begin{matrix} \underline{x}&\underline{y}\\ 1& 6\\ 2& 4\\ 3& 7\\ 4&B \\ A& 9 \end{matrix}$

Statement 1: A = 5

Statement 2: B = 4

Possible Answers:

EITHER statement ALONE is 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.

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

Correct answer:

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

Explanation:

To prove that a relation is a function, you must prove that no -coordinate is matched with more than one -coordinate. Statement 1 proves that this is true. Statement 2 is irrelevant; it is possible for more than one -coordinate to be matched with the same -coordinate in a function.

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Example Question #8 : Dsq: Understanding Functions

This relation has five different ordered pairs: is it a function?

$\begin{matrix} \underline{x}&\underline{y}\\ 1& 6\\ 2& 4\\ 3& 7\\ 4&B \\ A& 9 \end{matrix}$

Statement 1: A = 3

Statement 2: B = 5

Possible Answers:

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

BOTH statements TOGETHER are insufficient 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:

To prove that a relation is a function, you must prove that no -coordinate is matched with more than one -coordinate. Statement 1 proves that this is false, since 3 is now matched with both 7 and 9. Statement 2 is irrelevant, since it does not prove or disprove this condition.

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Example Question #9 : Dsq: Understanding Functions

Define an operation $\diamond$ as follows:

For any real numbers A,B ,

$A \diamond B = A^{2}-B$

Evaluate $5 \diamond 0 + 0 \diamond 5$

Possible Answers:

Undefined

Correct answer:

Explanation:

$5 \diamond 0 = 5^{2}-0 = 25$

$0\diamond 5 = 0^{2}-5 = -5$

$5 \diamond 0 + 0 \diamond 5 = 25 + (-5) = 20$

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GMAT Math : DSQ: Understanding functions

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Example Question #1 : Dsq: Understanding Functions

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Example Question #1 : Dsq: Understanding Functions

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Example Question #5 : Dsq: Understanding Functions

Example Question #3 : Dsq: Understanding Functions

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