GMAT Math : DSQ: Understanding powers and roots

Study concepts, example questions & explanations for GMAT Math

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Example Questions

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Example Question #1 : Dsq: Understanding Powers And Roots

 is a real number. Is  positive, negative, or zero?

Statement 1: 

Statement 2: 

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.

EITHER 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:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

If , then , and , so  must be positive.

If , then . and , so again,  must be positive. Either statement is enough to answer the question in the affirmative.

Example Question #3392 : Gmat Quantitative Reasoning

Simplify this expression as much as possible:

Possible Answers:

The expression is already simplified

Correct answer:

Explanation:

Example Question #3393 : Gmat Quantitative Reasoning

Imagine an integer  such that the units digit of is greater than 5. What is the units digit of ?

(1) The units digit of  is the same as the units digit of .

 

(2) The units digit of  is the same as the units digit of .

Possible Answers:

EACH statement ALONE is sufficient to answer the question.

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

BOTH statements TOGETHER are not sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but the other statement alone is not sufficient.

Statement 2 ALONE is sufficient to answer the question, but the other statement alone is not sufficient.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but the other statement alone is not sufficient.

Explanation:

(1) The only single-digit integer greater than 5 whose unit digit of its square term is equal to itself is 6. This statement is sufficient.

 

(2) There are two single-digit integers where the unit digit of the cubed term is equal to the integer itself: 6 and 9. This statement is insufficient.

Example Question #2 : Dsq: Understanding Powers And Roots

What is the value of twelve raised to the fourth power?

 

Possible Answers:

Correct answer:

Explanation:

"Twelve raised to the fourth power" is 124.  If you can translate the words into their mathematical counterpart, you're done, because the actual calculation should be done by your calculator. It will tell you that . There is not enough time on the test for you to try to do this by hand.

Example Question #3395 : Gmat Quantitative Reasoning

Calculate the fifth root of :

(1) The square root of  is .

(2) The tenth root of  is .

Possible Answers:

Statement (2) ALONE is sufficient, but Statement (1) ALONE is not sufficient.

Both statements TOGETHER are not sufficient.

Both statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

Statement (1) ALONE is sufficient, but Statement (2) ALONE is not sufficient.

Each statement ALONE is sufficient.

Correct answer:

Each statement ALONE is sufficient.

Explanation:

Using Statement (1):

Statement (1) ALONE is SUFFICIENT.

Using Statement (2):

Statement (2) ALONE is SUFFICIENT.

Therefore EACH  Statement ALONE is sufficient.

Example Question #3 : Dsq: Understanding Powers And Roots

 is a positive real number. True or false:  is a rational number.

Statement 1:  is an irrational number.

Statement 2:  is an irrational number.

Possible Answers:

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.

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:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

An integer power of a rational number, being a product of rational numbers, must itself be rational. Either statement alone asserts that such a power is irrational, so conversely, either statement alone proves  irrational.

Example Question #4 : Dsq: Understanding Powers And Roots

. True or false:  is rational.

Statement 1:  is rational.

Statement 2:  is rational.

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 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.

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:

Statement 1 alone is not enough to prove  is or is not rational. Examples:

If , then 

If , then 

In both cases,  is rational, but in one case,  is rational and in the other,  is irrational.

A similar argument demonstrates Statement 2 to be insufficient.

 

Assume both statements are true.  and  are rational, so their difference is as well:

 is rational, so by closure under division,  is rational. 

Example Question #5 : Dsq: Understanding Powers And Roots

. True or false:  is rational.

Statement 1:  is irrational.

Statement 2:  is rational.

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 insufficient 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.

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 alone is not enough to prove  rational or irrational. Examples:

If , then

If  , then 

In both cases,  is irrational, but in only one case,  is rational.

 

Assume Statement 2 alone.  is rational, so, by closure of the rational numbers under multiplication,

 is rational. The rationals are closed under addition, so the sum

is rational.

Example Question #3 : Dsq: Understanding Powers And Roots

 is a positive real number. True or false:  is a rational number.

Statement 1:  is irrational.

Statement 2:  is irrational.

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:

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

Explanation:

If  is rational, then, since the product of two rational numbers is rational,  is rational. If Statement 1 alone is assumed, then, since  is irrational,  must be irrational.

 

Assume Statement 2 alone, and note that

In other words,  is the square root of . Since both rational and irrational numbers have irrational square roots,  being irrational does not prove or disprove that  is rational.

Example Question #6 : Dsq: Understanding Powers And Roots

 is a positive real number. True or false:  is a rational number.

Statement 1:  is a rational number.

Statement 2:  is a rational number.

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.

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. 

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 alone provides insufficient information.  is a number with a rational cube root, , and a rational square root, .  is a number with a rational cube root, , but an irrational square root.

Now assume Statement 2 alone.

In other words, is the square of . The rational numbers are closed under multiplication, so if  is rational,   is rational.

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