Source: Official Guide Revised GRE 1st Ed. Part 8; Section 6; #16

2

The integer v is greater than 1

The integer v is greater than 1. If v is the square of an integer, which of the following numbers must also be the square of an integer? Indicate all such numbers.

3 Explanations

1

Rayssa Brandao

If v is an integer, shouldn't v squared be also an integer and thus C) would be correct?

Aug 8, 2018 • Comment

Sam Kinsman, Magoosh Tutor

Hi Rayssa,

You're right that v^2 would also be an integer. But note that the question is not asking us for answer choices that are an integer. The question is asking us for numbers that "must also be the square of an integer." And although C is an integer, it's not necessarily the square of an integer.

Aug 9, 2018 • Reply

1

Jorge Isaac Cordero Enriquez

Hi,

There are some questions that ask "which ... *could* be..." and some others ask "which...*must* be...". As I understand it, when the question asks for a *could*, it is enough if we find a particular case that works, however, if the question asks for a *must*, then it must hold true for all cases.

This question asks for a *must*. How can we assure it holds true for all cases if we just tried with v=4?

Thanks :)

Oct 5, 2017 • Comment

Jonathan , Magoosh Tutor

Hi Jorge,
We prove algebraically that A and B must be true. 4 is just an example.
We know v is an integer from the question.
A) 81v = (9v)^2 and 9v must be an integer. So (A) must be correct.

B) We should know our FOIL identities cold, including (a + b)^2 = a^2 + 2ab + b^2

So We can see that (B) factors to

(5v + 1)^2 and we know 5v is an integer so therefore 5v + 1 is also an integer.

So (B) is correct.

For (C), we could see that we can't factor the expression so therefore there is no algebraic reason that it must be a square . To be sure, we can find one example for which it's not a square. If v = 4 The expression equals 75 which is not the square of an integer. So we know (C) is not correct.

Oct 30, 2017 • Reply

5

Gravatar Chris Lele, Magoosh Tutor

Oct 11, 2012 • Comment

Bethany D Zacharias

Thank you for all of these explanation videos. The GRE answer key is not nearly as easy to understand.

Oct 14, 2013 • Reply

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