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# If a and b are prime numbers, which of the following must also be a pr

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Math Expert
Joined: 02 Sep 2009
Posts: 51215
If a and b are prime numbers, which of the following must also be a pr  [#permalink]

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25 Jul 2018, 23:40
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Difficulty:

5% (low)

Question Stats:

85% (00:52) correct 15% (01:49) wrong based on 66 sessions

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If a and b are prime numbers, which of the following must also be a prime number?

A. a + b

B. a – b

C. $$(a^2b)(\frac{1}{ab})$$

D. $$(a^2b^2)(\frac{1}{ab})$$

E. $$a^2b^2 – ab$$

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Re: If a and b are prime numbers, which of the following must also be a pr  [#permalink]

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25 Jul 2018, 23:51
1
Bunuel wrote:
If a and b are prime numbers, which of the following must also be a prime number?

A. a + b

B. a – b

C. $$(a^2b)(\frac{1}{ab})$$

D. $$(a^2b^2)(\frac{1}{ab})$$

E. $$a^2b^2 – ab$$

+1 for C.

= ( a^2 * b ) / a*b
= a (which itself is a prime number)

Hence, C.
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If a and b are prime numbers, which of the following must also be a pr  [#permalink]

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26 Jul 2018, 00:48
Bunuel wrote:
If a and b are prime numbers, which of the following must also be a prime number?

A. a + b

B. a – b

C. $$(a^2b)(\frac{1}{ab})$$

D. $$(a^2b^2)(\frac{1}{ab})$$

E. $$a^2b^2 – ab$$

Option C:

For any value this option is correct. Let's analyze ;

($$a)^2 b / ab$$

= $$\frac{a*a*b}{ab}$$

= a

we know a is prime . Stated in the question .

Anyone can plug the values too. Same result will be achieved .
Target Test Prep Representative
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Posts: 2830
Re: If a and b are prime numbers, which of the following must also be a pr  [#permalink]

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30 Jul 2018, 10:53
Bunuel wrote:
If a and b are prime numbers, which of the following must also be a prime number?

A. a + b

B. a – b

C. $$(a^2b)(\frac{1}{ab})$$

D. $$(a^2b^2)(\frac{1}{ab})$$

E. $$a^2b^2 – ab$$

Since (a^2 x b) x 1/ab = a, and a is prime, (a^2 x b) x 1/ab must result in a prime number.

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Re: If a and b are prime numbers, which of the following must also be a pr &nbs [#permalink] 30 Jul 2018, 10:53
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