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- aneesh.kg
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The expression resembles this form:
(a - b)^2
where
a = (7 + (29)^0.5)^0.5
b = (7 - (29)^0.5)^0.5
(a - b)^2
a^2 + b^2 - 2ab
a^2 = 7 + (29)^0.5
b^2 = 7 - (29)^0.5
2ab = 2*[(7 + (29)^0.5)(7 - (29)^0.5)]^0.5
= 2*[20]^0.5
= 4*[5]^0.5
So (a - b)^2
= 7 + 7 - 4[5]^0.5
= 14 - 4[5]^0.5
[spoiler](C)[/spoiler] is the answer
(a - b)^2
where
a = (7 + (29)^0.5)^0.5
b = (7 - (29)^0.5)^0.5
(a - b)^2
a^2 + b^2 - 2ab
a^2 = 7 + (29)^0.5
b^2 = 7 - (29)^0.5
2ab = 2*[(7 + (29)^0.5)(7 - (29)^0.5)]^0.5
= 2*[20]^0.5
= 4*[5]^0.5
So (a - b)^2
= 7 + 7 - 4[5]^0.5
= 14 - 4[5]^0.5
[spoiler](C)[/spoiler] is the answer
Last edited by aneesh.kg on Sun May 06, 2012 11:53 am, edited 1 time in total.
Aneesh Bangia
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Wrong! You missed the whole root!!!aneesh.kg wrote:The expression resembles this form:
(a - b)^2
where
a = (7 + (29)^0.5)
b = (7 - (29)^0.5)
(a - b)^2
a^2 + b^2 - 2ab
a^2 = 7 + (29)^0.5
b^2 = 7 - (29)^0.5
2ab = 2*(7 + (29)^0.5)(7 - (29)^0.5)
= 2*(49 - 29) = 40
So (a - b)^2
= 7 + 7 - 40
= - 26
[spoiler](A)[/spoiler] is the answer
- aneesh.kg
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Ah, Yes I did. Thanks.
Corrected the post above.
Corrected Solution:
The expression resembles this form:
(a - b)^2
where
a = (7 + (29)^0.5)^0.5
b = (7 - (29)^0.5)^0.5
(a - b)^2
a^2 + b^2 - 2ab
a^2 = 7 + (29)^0.5
b^2 = 7 - (29)^0.5
2ab = 2*[(7 + (29)^0.5)(7 - (29)^0.5)]^0.5
= 2*[20]^0.5
= 4*[5]^0.5
So (a - b)^2
= 7 + 7 - 4[5]^0.5
= 14 - 4[5]^0.5
[spoiler](C)[/spoiler] is the answer
Corrected the post above.
Corrected Solution:
The expression resembles this form:
(a - b)^2
where
a = (7 + (29)^0.5)^0.5
b = (7 - (29)^0.5)^0.5
(a - b)^2
a^2 + b^2 - 2ab
a^2 = 7 + (29)^0.5
b^2 = 7 - (29)^0.5
2ab = 2*[(7 + (29)^0.5)(7 - (29)^0.5)]^0.5
= 2*[20]^0.5
= 4*[5]^0.5
So (a - b)^2
= 7 + 7 - 4[5]^0.5
= 14 - 4[5]^0.5
[spoiler](C)[/spoiler] is the answer
Aneesh Bangia
GMAT Math Coach
[email protected]
GMATPad:
Facebook Page: https://www.facebook.com/GMATPad
GMAT Math Coach
[email protected]
GMATPad:
Facebook Page: https://www.facebook.com/GMATPad
- GMATGuruNY
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The square of a value must be nonnegative.What is the value of [√(7+√29) - √(7-√29)]²?
-26
2√29
14-4√5
14
14+4√5
Eliminate A.
Ballpark.
[√(7+√29) - √(7-√29)]²
≈ [√(7+6) - √(7-6)]² Since √25=5 and √36=6, √29 is between 5 and 6.
≈ (√13 - 1)²
≈ 3² Since √9=3 and √16=4, √13 is between 3 and 4.
≈ 9.
Since we overestimated the value of √13-1, the correct answer must be less than 9.
Only answer choice C works:
14-4√5 ≈ 14-8 ≈ 6.
The correct answer is C.
If we couldn't ballpark because the answer choices were closer in value, we could use the following approach.
Remembering that (a-b)² = a²-2ab+b², use substitution:
Let x=7 and y=√29.
Substituting x=7 and y=√29 into [√(7+√29) - √(7-√29)]², we get:
(√(x+y) - √(x-y))²
= (x+y) - 2√((x+y)(x-y)) + (x-y)
= 2x - 2√(x²-y²)
= 2*7 - 2√(7² - 29)
= 14 - 2√20
= 14 - 4√5.
The correct answer is C.
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Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3