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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## root tagged by: Brent@GMATPrepNow ##### This topic has 3 expert replies and 0 member replies ## root E: 9 $$\sqrt[4]{\frac{\left(99^2+101^2\right)}{2}-1}$$ A: 15 B: 12 C: 11 D: 10 OA is d. Can some experts help me with this? How do i come up with the correct approach to solve this question? Thanks ### GMAT/MBA Expert Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 122 members Upvotes: 1153 GMAT Score: 770 Roland2rule wrote: E: 9 $$\sqrt[4]{\frac{\left(99^2+101^2\right)}{2}-1}$$ A: 15 B: 12 C: 11 D: 10 OA is d. Can some experts help me with this? How do i come up with the correct approach to solve this question? Thanks 99^2 = (100-1)^2 = 100^2 - 200 + 1 = 10^4 -200 + 1 101^2 = (100+1)^2 = 100^2 + 200 + 1 = 10^4 + 200 + 1 So 99^2 + 100^2 = 10^4 - 200 + 1 + 10^4 + 200 + 1 = 2*10^4 + 2 And [(99^2 + 100^2 )/2] - 1 = [(2*10^4 + 2)/2] - 1 =( 10^4 + 1) -1 = 10^4 All that's left to do is to take the fourth root of 10^4, which is just 10. The answer is D _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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### GMAT/MBA Expert

GMAT Instructor
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Roland2rule wrote:
$$\sqrt[4]{\frac{\left(99^2+101^2\right)}{2}-1}$$
A: 15
B: 12
C: 11
D: 10
E: 9
We might also apply some approximation.
Notice that (99Â² + 101Â²)/2 represents the AVERAGE of 99Â² and 101Â²
The average of 99Â² and 101Â² is going to be approximately equal to 100Â²
So, the fourth root of (99Â² + 101Â²)/2 - 1 â‰ˆthe fourth root of 100Â² - 1
Next, since 100Â² is equal to 10,000, subtracting 1 from has little consequence. So, let's ignore it

So, $$\sqrt[4]{\frac{\left(99^2+101^2\right)}{2}-1}$$ â‰ˆ the fourth root of (100Â² - 1)
â‰ˆ fourth root of (100Â² - 1)
â‰ˆ fourth root of (100Â²)
â‰ˆ fourth root of (10,000)
â‰ˆ 10

Answer: D

Cheers,
Brent

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### GMAT/MBA Expert

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Hi Roland2rule,

In complex-looking calculations, if you find that you don't see an immediate pattern that will help you to 'break down' the math, sometimes you just have to think about what the numbers represent in real simple terms.

I'm going to start with the squared terms: 99^2 and 101^2 (since we're asked to add these numbers together).

99^2 is like saying "ninety-nine 99s" --> imagine a big row of 99s that you have to add up....but don't add them up yet....
101^2 is like saying "one hundred one 101s" --> it's the same idea...a big row of 101s that you have to add up....

Now, take ONE 99 and add it to ONE 101 and you get 200. How many of those 200s do you have here?

You have ninety-nine 200s with two extra 101s left over....This gives us....

99(200) + 2(101) = 19,800 + 202 = 20,002

Next, we're asked to divide this sum by 2 and then subtract 1...

20,002/2 = 10,001

10,001 - 1 = 10,000

Finally, we're asked to take the fourth root (or quad-root) of 10,000....

since 10^4 = 10,000.....the fourth root of 10,0000 = 10

Final Answer: D

GMAT assassins aren't born, they're made,
Rich

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