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# root[3*root{80}+3/(9+4*root{5})]=?

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26 Jan 2012, 06:30
5
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Difficulty:

35% (medium)

Question Stats:

76% (02:29) correct 24% (02:08) wrong based on 279 sessions

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$$\sqrt{3*\sqrt{80}+\frac{3}{9+4*\sqrt{5}}}=?$$

A) $$\sqrt{3*\sqrt{5}}$$

B) 3

C) $$3*\sqrt{3}$$

D) $$3+2*\sqrt{5}$$

E) $$9+4*\sqrt{5}$$
[Reveal] Spoiler: OA
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26 Jan 2012, 06:59
3
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Expert's post
LM wrote:
$$\sqrt{3*\sqrt{80}+\frac{3}{9+4*\sqrt{5}}}=?$$

A) $$\sqrt{3*\sqrt{5}}$$

B) 3

C) $$3*\sqrt{3}$$

D) $$3+2*\sqrt{5}$$

E) $$9+4*\sqrt{5}$$

$$\sqrt{3\sqrt{80}+\frac{3}{9+4\sqrt{5}}}=\sqrt{12\sqrt{5}+\frac{3}{9+4\sqrt{5}}*\frac{9-4\sqrt{5}}{9-4\sqrt{5}}}=\sqrt{12\sqrt{5}+\frac{3(9-4\sqrt{5})}{81-80}}=\sqrt{12\sqrt{5}+27-12\sqrt{5}}=\sqrt{27}=3\sqrt{3}$$.

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26 Jan 2012, 07:00
By taking reciprocals we can solve this question. Ans is C
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12 Aug 2012, 09:56
Thanks Bunuel -- You Are The Man!!
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12 Aug 2012, 11:29
This can be solved by taking the denominator as 9+4\sqrt{5} and then taking this as the common factor. We get the the ans as C.
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16 Aug 2012, 23:16
This kind of problems always seemed to me very scary and requires a lot of calculation, but later i realised that GMAT never asks something that you need to calculate a lot, so one needs to look for some pattern or similar numbers/sets. In our case, we look at denominator 9+\sqrt{5} and 3\sqrt{80}, so 80 is 2^4*5, which means 4\sqrt{5}, from here we feel that numerator and denominator could be reduced. The rest is just calculations. In my opinion the most crucial part is this one.
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07 Apr 2013, 10:59
Hi,

I was going through MGMAT CAT and found the below question.

{(80)^1/3 + 3 /(9 + 4 (5)^1/2)}^1/2

I am unable to figure out how the answer is coming. Could anyone please help me with this. (Attached the solution as a image)

I couldn't post other options as the page is blurred.
Attachments

Sol.JPG [ 21.08 KiB | Viewed 5032 times ]

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Re: Que from MGMAT - [#permalink]

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07 Apr 2013, 11:07
Lets analyze the first part $$3\sqrt{80} = 3\sqrt{5*16} = 3*4\sqrt{5}$$
The second term: Denominator $$(9+4\sqrt{5})*(9-4\sqrt{5})=9^2-4^2*5=1$$ Rule: (x+y)(x-y)=x^2-y^2
The second term: Numerator $$3*(9-4\sqrt{5})=27-12\sqrt{5}$$
Now putting all in one:
$$\sqrt{(}12\sqrt{5}+27-12\sqrt{5})=$$
$$\sqrt{27}=\sqrt{3*3^2}=3\sqrt{3}$$

Hope it's clear now
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Re: Que from MGMAT - [#permalink]

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07 Apr 2013, 22:36
Expert's post
saratchandra wrote:
Hi,

I was going through MGMAT CAT and found the below question.

{(80)^1/3 + 3 /(9 + 4 (5)^1/2)}^1/2

I am unable to figure out how the answer is coming. Could anyone please help me with this. (Attached the solution as a image)

I couldn't post other options as the page is blurred.

Merging similar topics. Please refer to the solution above.

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Re: Que from MGMAT - [#permalink]

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14 Apr 2013, 08:16
Zarrolou wrote:
Lets analyze the first part $$3\sqrt{80} = 3\sqrt{5*16} = 3*4\sqrt{5}$$
The second term: Denominator $$(9+4\sqrt{5})*(9-4\sqrt{5})=9^2-4^2*5=1$$ Rule: (x+y)(x-y)=x^2-y^2
The second term: Numerator $$3*(9-4\sqrt{5})=27-12\sqrt{5}$$
Now putting all in one:
$$\sqrt{(}12\sqrt{5}+27-12\sqrt{5})=$$
$$\sqrt{27}=\sqrt{3*3^2}=3\sqrt{3}$$

Hope it's clear now

thank you!! .. that 3 is so small that i took cube root 80...
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Re: Que from MGMAT - [#permalink]

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14 Apr 2013, 08:17
Bunuel wrote:
saratchandra wrote:
Hi,

I was going through MGMAT CAT and found the below question.

{(80)^1/3 + 3 /(9 + 4 (5)^1/2)}^1/2

I am unable to figure out how the answer is coming. Could anyone please help me with this. (Attached the solution as a image)

I couldn't post other options as the page is blurred.

Merging similar topics. Please refer to the solution above.

thanks for merging with the old thread.. i couldn't understand how to search, so posted it. I will try to avoid that in future
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30 Dec 2014, 00:22
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02 Jan 2015, 01:12
$$\sqrt{3*\sqrt{80}+\frac{3}{9+4*\sqrt{5}}}$$

$$= \sqrt{3 * (4\sqrt{5}+\frac{1}{9+4*\sqrt{5}})}$$

$$= \sqrt{3 * \frac{36\sqrt{5} + 80 + 1}{9+4*\sqrt{5}}}$$

$$= \sqrt{3 * 9 * \frac{9+4*\sqrt{5}}{9+4*\sqrt{5}}}$$

$$= 3 \sqrt{3}$$
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04 Apr 2015, 07:52
LM wrote:
$$\sqrt{3*\sqrt{80}+\frac{3}{9+4*\sqrt{5}}}=?$$

A) $$\sqrt{3*\sqrt{5}}$$

B) 3

C) $$3*\sqrt{3}$$

D) $$3+2*\sqrt{5}$$

E) $$9+4*\sqrt{5}$$

Though, I am quite comfortable with the method mentioned by Bunuel, I found an alternative way by The Economist.

$$9 + 4*\sqrt{5}$$ will be approx. equal to 9 + 4 x 2 = 17. Hence 3/ 17 will be quite less to contribute towards the value of expression.

$$3\sqrt{80}$$ is approx. 3 x 9 = 27. now $$\sqrt{27}$$ will be something more than 5.

Now coming to options:

(A)$$\sqrt{3*\sqrt{5}}$$ is approx $$\sqrt{6}$$ which is quite less than 5. Rejected.
(B) Rejected.
(C) 3 x 1.732 = 5.1 , which is in our desired range.
(D) 3 + 4 =7. Rejected
(E) 9 + 4 x 2 = 17. Rejected.

Hence the correct ans is (C).

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09 Jun 2016, 02:17
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: root[3*root{80}+3/(9+4*root{5})]=?   [#permalink] 09 Jun 2016, 02:17
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