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f(f(x)) = \sqrt{([square_root](x)})[/square_root]

f(f(x)) = x^1/4 = 1/(x)^4

We will put in options and see which one can be depicted as a power of 4.

A - 0.25 = 1/4 = 1/2^2. ELIMINATE

B - 0.04 = 1/25 = 1/5^2. ELIMINATE

C - 0.016 = 2/125 = 2/5^3. ELIMINATE

D - 0.008 = 1/125 = 1/5^3. ELIMINATE

E - 0.0016 = 1/625 = 1/5^4. ANSWER

FINAL ANSWER - Option E

Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of Prizes

If x is a positive number, \(f(x) = \sqrt{x}\), and \(f(f(x))\) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

 


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Sure! Let’s simplify the problem step by step:
Problem Recap:
  • Function: f(x) = √x
  • Given: f(f(x)) is the reciprocal of a prime number.
  • Goal: Find which value of xx fits this condition.
Steps to Solve:
  1. Find f(f(x)):
    f(f(x)) = f(x) =√√x = x^1/4
    So, x^1/4 is the reciprocal of a prime number.
  2. Set Up the Equation:
    x^1/4 = 1/p
    where p is a prime number.
  3. Solve for xx:
    x = (1/p)^4 = 1/p^4
    This means xx must be 1/p^4.
  4. Check the Options:
    • Option E: 0.0016
      0.0016 = 1/625 =1/5^4
      • Here, p = 5, which is a prime number.
      • So, x = 1/5^4 = 0.0016 fits the condition.

Answer:
E. 0.0016
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ff(x)=root of root (x) which would be equal to reciprocal of prime number,
so on observing the options we can assume the prime no to be 2 or 5
so
x raised to the power 1/4=1/2 or 1/5,
on squaring both sides and
on solving further we can get x=0.0016 which is matching our answer.


Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of Prizes

If x is a positive number, \(f(x) = \sqrt{x}\), and \(f(f(x))\) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 

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f(f(x)) = x^(1/4)

Expressing numbers as fractions, the only number that is rational is

0.0016^(1/4)=(2^4/10^4)^(1/4)=1/5, whose reciprocal is 5, that is a prime number.

Answer E
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If f(x) = x^1/2 then f (f (x)) = x^1/4
If x^1/4 = 1/p then value of x could be x = 1/p^1/4
Evaluating the answer options:
A. 0.25 = 25/100 = 1/4. 4 is not the 4th root of any prime number
B. 0.04 = 4/100 = 1/25. 25 is not the 4th root of any prime number
C. 0.016 = 16/1000 = 62.5. 62.5 is not the 4th root of any prime number
D. 0.008 = 8/1000 = 125. 125 is not the 4th root of any prime number
E. 0.0016 = 16/10000= 625. 625 is not the 4th root of 5
Therefore E is the the answer
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Quote:
If x is a positive number, f(x)=sqrt (x), and f(f(x)) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

so f(f(x)) = f(sqrt(x)) = x^(1/4) = 1/p, where p = prime number
-> x =1/(p^4) => p = (1/x)^(1/4)

Turned answer choices into fractions to see which ones would yield an integer when taken root of 4. Only E satisfies, as 0.0016 = (2/10)^4 => p = 10/2 = 5, a prime number
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IMO E


The best way to do these kind of question is by using options
got from A to E and you willl find that only E satsisfy the condition

with Prime Number as 5 hence E
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Looking at the options, we can clearly see that E when put in the function gives the value of 0.2 which is reciprocal of prime number 5

Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of Prizes

If x is a positive number, \(f(x) = \sqrt{x}\), and \(f(f(x))\) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 

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f(x) = √x
And we need a number such that √√x is a reciprocal of a positive number => x should be a number such that x^(0.25) is a reciprocal of a positive number

Please note that only option e can be written in the form => (2^4)/ (10^4)
If x = (2^4)/ (10^4) => √√x = 2/10 => 1/5
That is 1/x = 5

Hence option e is the answer
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It is given that

\(\\
f(x) = \sqrt{x}\\
\\
f(f(x)) = f(\sqrt{x}) = x^{1/4}\\
\)

It is also given that \(f(f(x))\) is the reciprocal of a prime number
\(\\
x^{1/4} = \frac{1}{p}\\
x = \frac{1}{p^4}\\
\)

Substituting to find the answer,
\(\\
p = 2, x = \frac{1}{2^4} = 0.625\\
p = 3, x = \frac{1}{3^4} = 0.012\\
p = 5, x = \frac{1}{5^4} = 0.0016\\
\)

Verifying,
If \( x = 0.0016\),
\(\\
f(x) = \sqrt{0.0016} = 0.04\\
f(f(x)) = 0.2 = 1/5\\
\)

5 is a prime number

Answer: E
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f(x) = √x
f(f(x)) = f(√x) = x ^0.25

Now we know that f(f(x) is reciprocal of a prime number so let it be y
x^0.25 = 1/y
x = 1/(y^4)

Now we substitute the values 1 by 1

x = 0.25
=> 0.25 = 1/4 = 1/y^4
which cannot be possible as y^4 is not equal to 4

x = 0.04
=> 0.04 = 1/25 = 1/y^4
which cannot be possible as y^4 is not equal to 25

x = 0.016
=> 0.016 = 1/625 = 1/y^4
which is possible as y^4can be equal to 625 for the value of 5 which is also a prime number

C. 0.016
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f(x)=x^(1/2)

f(f(x))=x^(1/4) , which is a reciprocal of a prime number(P)
1/P=x^(1/4)
P={1/x}^(1/4)

Substituting all given values of x in answer choices

A. 0.25
{1/0.25}^(1/4)={4}^(1/4)=2^(1/2) , which is not a prime number

B. 0.04
{1/0.04}^(1/4)={25}^(1/4)=5^(1/2) , which is not a prime number


C. 0.016
{1/0.016}^(1/4)={125/2}^(1/4) , which is not a prime number


D. 0.008
{1/0.008}^(1/4)=125^(1/4) , which is not a prime number


E. 0.0016
{1/0.0016}^(1/4)={625}^(1/4)=5 ,which is a prime number

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f(f(x)) =\( x^\frac{1}{4}\)

Answer must be a fourth power
Only option that satisfied this - 0.0016 = \(2^4 * 0.1^4\)
Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of Prizes

If x is a positive number, \(f(x) = \sqrt{x}\), and \(f(f(x))\) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 

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We need a option such that sqrt(sqrt(x)) is a reciprocal of a positive number
a) 25/100 = 5^2/10^2
b) 4/100 = 2^2/10^2
c) 16/1000 = 4^2/10^3
d) 8/1000 = 2^3/10^3
e) 16/10000 = 2^4/10^4 => sqrt(sqrtx)) = 2/10 => CORRECT
Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of Prizes

If x is a positive number, \(f(x) = \sqrt{x}\), and \(f(f(x))\) is the reciprocal of a prime number, which of the following could be the value of x?

A. 0.25
B. 0.04
C. 0.016
D. 0.008
E. 0.0016

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 

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In this case, we're looking for the root of fourth power of X, which must have a prime number in the denominator.

From the fractions provided, only \( 0.0016=\frac{1}{625}=\frac{1}{5^4}\) gives us the result, as the denominator will be 5 (prime).
The answer is therefore E.
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