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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: 47918
If a and b are prime numbers, which of the following must also be a pr  [#permalink]

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26 Jul 2018, 00:40
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5% (low)

Question Stats:

88% (00:47) correct 12% (01:42) wrong based on 51 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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Location: India
Concentration: Operations, Marketing
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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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26 Jul 2018, 00:51
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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, 01:48
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 .
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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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30 Jul 2018, 11:53
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, 11:53
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