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Which of the following is equal to \(\frac{\sqrt{6} - \sqrt{2}}{\sqrt{6} + \sqrt{2}}\)?

A. \(2+\sqrt{3}\)
B. \(2-\sqrt{3}\)
C. \(\frac{\sqrt{2}}{2}\)
D. \(\frac{\sqrt{6} + \sqrt{2}}{4}\)
E. \(\frac{7 - 2\sqrt{6}}{5}\)

We can make the denominator “disappear” if we remove the radicals from the denominator.

How can we remove those radicals without changing the value of the fraction? By multiplying the fraction by a corresponding fraction that is equal to one:

[(√6 – √2)/(√6 + √2)] × [(√6 – √2)/(√6 – √2)] =

[(√6 – √2)(√6 – √2)]/[(√6 + √2)(√6 – √2)] =

(6 – 2√12 + 2)/(6 – 2) =

(8 – 4√3)/4 =

2 – √3

Answer: B
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All those struggling with the algebraic approach can try the arithmetic approach.

\(\frac{\sqrt{6} - \sqrt{2}}{\sqrt{6} + \sqrt{2}}\)

\(~\frac{2.4-1.4}{2.4+1.4} = \frac{1}{3.8} ~ \frac{1}{4}\)

Find an answer choice which nearly equals 1/4 or 0.25

A. \(2+\sqrt{3}\) \(= 2+~1.73 = ~3.7\)

B. \(2-\sqrt{3}\) \(= 2- 1.73 = 0.27\)

C. \(\frac{\sqrt{2}}{2}\) \(= \frac{1.4}{2}\)

D. \(\frac{\sqrt{6} + \sqrt{2}}{4}\) \(= \frac{2.4+1.4}{4} = 3.8/4 =1\)

E. \(\frac{7 - 2\sqrt{6}}{5}\) = \(\frac{7 - 2*2.4}{5}\) \(= \frac{2.2}{5} ~0.5\)

Answer: B­
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ravjaz
Would appreciate someone's help on this!


Attachment:
GMAT-Club-Forum-hwso6mwi.png
You need to rationalize the denominator so that we wont have any irrational number there.

Now, How to do that...

\sqrt{6} + \sqrt{2}, Can be rational if we multiply it by \sqrt{6} - \sqrt{2},

So if we multiply both numerator and denominator with the same then there won't be any change.

\((\sqrt{6} - \sqrt{2}/ \sqrt{6} + \sqrt{2} )* (\sqrt{6} - \sqrt{2} / \sqrt{6} - \sqrt{2})\)

If you simplify this, It will become \( (\sqrt{6} - \sqrt{2 } )^2 * 1/4\\
\)

SO now you can do what ever you need from here on, Key here is to rationaize.
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Thanks for this, not sure if you might know but I noticed there are questions where we square the entire fraction instead, how do i know if i should rationalise first or square? Is it always rationalise first?
HarshaBujji
ravjaz
Would appreciate someone's help on this!


Attachment:
GMAT-Club-Forum-hwso6mwi.png
You need to rationalize the denominator so that we wont have any irrational number there.

Now, How to do that...

\sqrt{6} + \sqrt{2}, Can be rational if we multiply it by \sqrt{6} - \sqrt{2},

So if we multiply both numerator and denominator with the same then there won't be any change.

\((\sqrt{6} - \sqrt{2}/ \sqrt{6} + \sqrt{2} )* (\sqrt{6} - \sqrt{2} / \sqrt{6} - \sqrt{2})\)

If you simplify this, It will become \( (\sqrt{6} - \sqrt{2 } )^2 * 1/4\\
\)

SO now you can do what ever you need from here on, Key here is to rationaize.
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ravjaz
Thanks for this, not sure if you might know but I noticed there are questions where we square the entire fraction instead, how do i know if i should rationalise first or square? Is it always rationalise first?
HarshaBujji
ravjaz
Would appreciate someone's help on this!


Attachment:
GMAT-Club-Forum-hwso6mwi.png
You need to rationalize the denominator so that we wont have any irrational number there.

Now, How to do that...

\sqrt{6} + \sqrt{2}, Can be rational if we multiply it by \sqrt{6} - \sqrt{2},

So if we multiply both numerator and denominator with the same then there won't be any change.

\((\sqrt{6} - \sqrt{2}/ \sqrt{6} + \sqrt{2} )* (\sqrt{6} - \sqrt{2} / \sqrt{6} - \sqrt{2})\)

If you simplify this, It will become \( (\sqrt{6} - \sqrt{2 } )^2 * 1/4\\
\)

SO now you can do what ever you need from here on, Key here is to rationaize.

We square to simplify - either to eliminate a square root or, in some cases, an absolute value. But here, squaring (√6 - √2)/(√6 + √2) gives (4 - √24)/(8 + √24), which isn’t helpful.

Also, for questions that ask for the value of an expression, such as this one, if you square to simplify, you’d need to take the square root again at the end to balance the operation.

On the other hand, the purpose of rationalization is to eliminate irrational expressions in the denominator and simplify the expression, which works well for this problem.

Check question involving rationalization of a fraction:

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