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Algebra

by sud21 » Fri Oct 09, 2015 8:03 pm
6/( 1/ √ 12 + 1/√ 13 ) =

a)6/5

b)5/6

c)5

d)2√3 +3 √2

e)6√3−6√ 2
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by GMATinsight » Fri Oct 09, 2015 8:58 pm
sud21 wrote:6/( 1/ √ 12 + 1/√ 13 ) =

a)6/5

b)5/6

c)5

d)2√3 +3 √2

e)6√3−6√ 2
BEST Method:

6/( 1/ √ 12 + 1/√ 13 ) = 6/(1/3.5 + 1/3.5) = 6/(0.3+0.3) = 6/0.6 = 10

Checking Options:

a)6/5 Close to 1.2 so nowhere close to 10 which is expected result of given expression

b)5/6 Close to 0.8 so nowhere close to 10 which is expected result of given expression

c)5 Nowhere close to 10 which is expected result of given expression

d)2√3 +3 √2 = 3.4+4.2=7.6 Closest Option

e)6√3−6√ 2 Close to 0 so nowhere close to 10 which is expected result of given expression

Answer: Option D
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by GMATinsight » Fri Oct 09, 2015 9:02 pm
GMATinsight wrote:
sud21 wrote:6/( 1/ √ 12 + 1/√ 13 ) =

a)6/5

b)5/6

c)5

d)2√3 +3 √2

e)6√3−6√ 2
BEST Method:

6/( 1/ √ 12 + 1/√ 13 ) = 6/(1/3.5 + 1/3.5) = 6/(0.3+0.3) = 6/0.6 = 10

Checking Options:

a)6/5 Close to 1.2 so nowhere close to 10 which is expected result of given expression

b)5/6 Close to 0.8 so nowhere close to 10 which is expected result of given expression

c)5 Nowhere close to 10 which is expected result of given expression

d)2√3 +3 √2 = 3.4+4.2=7.6 Closest Option

e)6√3−6√ 2 = 6(1.7-1.4) Close to 1.8 so nowhere close to 10 which is expected result of given expression

Answer: Option D
Although I have eliminated other options to find best suitable option but these options still look Incorrect as the answer is nowhere close to 10 while checking any option
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by GMATGuruNY » Sat Oct 10, 2015 1:40 am
sud21 wrote:6/(1/√ 12 + 1/√ 13 ) =

a)6/5

b)5/6

c)5

d)2√3 + 3√2

e)6√3 − 6√2
6/(1/√12 + 1/√13 ) = 6/(positive fraction) = MORE THAN 6.
Of the five answer choices, only D Is greater than 6.

The correct answer is D.
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by Brent@GMATPrepNow » Sat Oct 10, 2015 7:01 am
sud21 wrote:6/( 1/ √ 12 + 1/√ 13 ) =

a)6/5

b)5/6

c)5

d)2√3 +3 √2

e)6√3−6√ 2
√9 = 3 and √16 = 4
√12 is BETWEEN √9 and √16, so let's say √12 = APPROXIMATELY 3.5
Likewise, √13 is BETWEEN √9 and √16, so let's say √12 = APPROXIMATELY 3.5 also
Aside: As I will soon show, we need not be more accurate than this.

So, 6/(1/√12 + 1/√13 ) ≈ 6/(1/3.5 + 1/3.5 )
≈ 6/(2/3.5)
≈ (6)(3.5/2)
≈ (3)(3.5)
≈ 10.5

Before, we check the answer choices, it's important to note that everyone should memorize the following roots for test day:
√2 ≈ 1.4
√3 ≈ 1.7
√3 ≈ 2.2

a)6/5 too far from 10.5. ELIMINATE

b)5/6 too far from 10.5. ELIMINATE

c)5 too far from 10.5. ELIMINATE

d)2√3 + 3√2 = 5√2 ≈ (5)(1.4) = 7 This is the best answer

e)6√3 - 6√2 = Some negative value. too far from 10.5. ELIMINATE

Answer: D
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