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root[3*root{80}+3/(9+4*root{5})]=? [#permalink]
 26 Jan 2012, 06:30
Question Stats:
69% (02:11) correct
30% (02:01) wrong based on 56 sessions
\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}
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By taking reciprocals we can solve this question. Ans is C
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Re: root[3*root{80}+3/(9+4*root{5})]=? [#permalink]
12 Aug 2012, 09:56
Thanks Bunuel -- You Are The Man!! 
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Re: root[3*root{80}+3/(9+4*root{5})]=? [#permalink]
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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Re: root[3*root{80}+3/(9+4*root{5})]=? [#permalink]
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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Hi, I was going through MGMAT CAT and found the below question. {(80)^1/3 + 3 /(9 + 4 (5)^1/2)}^1/2 Answer C): 3 (3)^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 1610 times ]
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Re: Que from MGMAT - [#permalink]
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]
07 Apr 2013, 22:36
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Re: Que from MGMAT - [#permalink]
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]
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
Answer C): 3 (3)^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. Also, please read carefully and follow: rules-for-posting-please-read-this-before-posting-133935.htmlthanks 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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Re: Que from MGMAT -
[#permalink]
14 Apr 2013, 08:17
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