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Re: Which of the following is equivalent to 21/6 ? [#permalink]

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27 Jun 2017, 02:41

Why in this case it's wrong to square all and remove the square root? In this question https://gmatclub.com/forum/2-240047.html I saw that it's feasible... why in this case it isn't?

Which of the following is equivalent to 21/6 ? [#permalink]

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27 Jun 2017, 03:34

MvArrow wrote:

Why in this case it's wrong to square all and remove the square root? In this question https://gmatclub.com/forum/2-240047.html I saw that it's feasible... why in this case it isn't?

Let a= \(\frac{\sqrt{21}}{6}\)

If you want to remove the square root, square both sides

Which of the following is equivalent to \(\frac{\sqrt{21}}{6}\)?

A. \(\frac{\sqrt{7}}{2}\)

B. 7/12

C. \(\sqrt{\frac{7}{2}}\)

D. \(\sqrt{\frac{7}{12}}\)

E. \(\frac{7\sqrt{3}}{2}\)

Why in this case it's wrong to square all and remove the square root? In this question https://gmatclub.com/forum/2-240047.html I saw that it's feasible... why in this case it isn't?

You can do this way but you should square the options too to compare apples with apples. When you square, the question becomes: which of the following is equivalent to \((\frac{\sqrt{21}}{6})^2=\frac{21}{36}=\frac{7}{12}\). Option D squared gives exactly this value: \((\sqrt{\frac{7}{12}})^2=\frac{7}{12}\)

Re: Which of the following is equivalent to 21/6 ? [#permalink]

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27 Jun 2017, 04:16

Bunuel wrote:

MvArrow wrote:

Which of the following is equivalent to \(\frac{\sqrt{21}}{6}\)?

A. \(\frac{\sqrt{7}}{2}\)

B. 7/12

C. \(\sqrt{\frac{7}{2}}\)

D. \(\sqrt{\frac{7}{12}}\)

E. \(\frac{7\sqrt{3}}{2}\)

Why in this case it's wrong to square all and remove the square root? In this question https://gmatclub.com/forum/2-240047.html I saw that it's feasible... why in this case it isn't?

You can do this way but you should square the options too to compare apples with apples. When you square, the question becomes: which of the following is equivalent to \((\frac{\sqrt{21}}{6})^2=\frac{21}{36}=\frac{7}{12}\). Option D squared gives exactly this value: \((\sqrt{\frac{7}{12}})^2=\frac{7}{12}\)

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