4
$\begingroup$

If $\sqrt{28x}$ is an integer is $\sqrt{7x}$ an integer? I have a book that says no, but I cannot think of an example of the contrary... Not looking for a full proof here just wanting to see a counterexample or logic showing that the book is wrong...

$\endgroup$
4
  • $\begingroup$ Notice that $\sqrt{28}=2\sqrt{7}$ $\endgroup$
    – user84413
    Commented Sep 3, 2015 at 23:20
  • 3
    $\begingroup$ If $2A$ is an integer, is $A$ an integer? $\endgroup$ Commented Sep 3, 2015 at 23:35
  • 1
    $\begingroup$ Just a comment: You say you want to "check your logic". But you haven't actually told us what your logic is, other than "I can't think of a counterexample." In general, "I can't think of a counterexample" is not good logic at all. $\endgroup$
    – mweiss
    Commented Sep 3, 2015 at 23:40
  • $\begingroup$ mweiss, great point. Sorry for the poor word choice. I've changed the question to better reflect what I mean. $\endgroup$
    – Padawan
    Commented Sep 3, 2015 at 23:45

4 Answers 4

10
$\begingroup$

I cannot think of an example of the contrary $\dots$ just wanting to see a counterexample

Would $~x=\dfrac1{28}~$ satisfy your curiosity ?

$\endgroup$
1
  • 2
    $\begingroup$ It is outright mind-boggling how my most basic answers are also the ones gathering the most upvotes. $\endgroup$
    – Lucian
    Commented Sep 4, 2015 at 2:46
4
$\begingroup$

Consider the first part $\sqrt{28 \, x} = \pm n$, $n$ being an integer. Then one finds $$x = \frac{n^{2}}{28}.$$ Now consider the second part $$\sqrt{7 \, x} = \sqrt{ \frac{7 \, n^{2}}{28} } = \sqrt{\frac{n^{2}}{4}} = \pm \frac{n}{2}.$$ This shows that if $n$ is not an even value then the value of $\sqrt{7 \, x}$ is not an integer.

$\endgroup$
2
  • 3
    $\begingroup$ Presumably you meant to write "an even integer" rather than "an even function", correct? $\endgroup$
    – mweiss
    Commented Sep 4, 2015 at 0:09
  • 3
    $\begingroup$ Why is it that the positive root of $7x$ can yield a negative number? Wouldn't it just be $\sqrt{7x}=\frac{n}{2}$? $\endgroup$
    – Ud779
    Commented Sep 4, 2015 at 0:14
3
$\begingroup$

Note that $\sqrt{28x}=\sqrt{4\times 7x}=2\sqrt{7x}$. Can you see what might happen so that $\sqrt{28x}$ is an integer while $\sqrt{7x}$ is not?

$\endgroup$
0
$\begingroup$

Suppose x is an integer.

Then 28 * x is always even, since 28 * x = 2 * 14x.

The square root of an even number, is never an odd number. (Check that odd * odd = odd)

So if you take examples where x is an integer, sqrt(28x) is never odd, and therefore there exists no counterexample.

So x must be a fraction to find a counterexample, (like 1/28 or 9/28).

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .