GRE Prep Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
If f(x) = f , which of the following is true for all values
[#permalink]
30 Jul 2018, 09:47
30 Jul 2018, 09:47
1
Expert Reply
8
Bookmarks
00:00
Question Stats:
60% (00:56) correct
39% (01:36) wrong based on 69 sessions
Hide Show timer Statistics
If \(\frac{1}{2}f(x) = f(\frac{x}{2})\) , which of the following is true for all values of f(x)?
(A) \(f(x) = 2x + 2\)
(B) \(f(x) = 13x\)
(C) \(f(x) = x^2\)
(D) \(f(x) = x - 10\)
(E) \(f(x) =\sqrt{x-4}\)
(A) \(f(x) = 2x + 2\)
(B) \(f(x) = 13x\)
(C) \(f(x) = x^2\)
(D) \(f(x) = x - 10\)
(E) \(f(x) =\sqrt{x-4}\)
Re: If f(x) = f , which of the following is true for all values
[#permalink]
11 Aug 2018, 13:28
11 Aug 2018, 13:28
This question was phrased in an odd way in my opinion, but what it's asking is which of the functions fulfills the constraint that 1/2f(x) = f(x/2)
Depending on your math background, you may now that scalars don't work the same way inside roots and exponents as they do outside, and scalars outside functions will affect constants inside function, which we don't want. If you don't have that math background, you can check with values. Here is a somewhat convoluted explanation without values.
We can quickly rule out any option with a constant, as the constant in 1/2f(x) will be affected by the division by 2, but the constant in f(x/2) will not.
Example: f(x) = 2x+2
1/2(2x+2) = x+1 != 2x/2 + 2 = f(x/2)
We can also rule out the square and the square root by noting that:
(x/2)^2 = x^2/4 != x^2/2
and
sqrt{x/2} != sqrt{x/4} = 1/2*sqrt{x}
In these cases, the constant scales with the square or the square root, which we don't want.
Simply put, we want a linear function without a constant, and the only option that fulfills that constraint is option B.
Or you can quickly hammer in the functions with the scalar inside and outside the function into your calculator. If you do it on this question, as 13x is an early option and only one option is correct, it would have saved you time. But it obviously depends on which order you pick the options in.
Depending on your math background, you may now that scalars don't work the same way inside roots and exponents as they do outside, and scalars outside functions will affect constants inside function, which we don't want. If you don't have that math background, you can check with values. Here is a somewhat convoluted explanation without values.
We can quickly rule out any option with a constant, as the constant in 1/2f(x) will be affected by the division by 2, but the constant in f(x/2) will not.
Example: f(x) = 2x+2
1/2(2x+2) = x+1 != 2x/2 + 2 = f(x/2)
We can also rule out the square and the square root by noting that:
(x/2)^2 = x^2/4 != x^2/2
and
sqrt{x/2} != sqrt{x/4} = 1/2*sqrt{x}
In these cases, the constant scales with the square or the square root, which we don't want.
Simply put, we want a linear function without a constant, and the only option that fulfills that constraint is option B.
Or you can quickly hammer in the functions with the scalar inside and outside the function into your calculator. If you do it on this question, as 13x is an early option and only one option is correct, it would have saved you time. But it obviously depends on which order you pick the options in.
Re: If f(x) = f , which of the following is true for all values
[#permalink]
12 Aug 2018, 07:19
12 Aug 2018, 07:19
2
Expert Reply
Explanation
The question is asking, “For which function is performing the function on x and THEN multiplying by \(\frac{1}{2}\) the equivalent of performing the function on \(\frac{1}{2}\) of x?”
The fastest method is to use logic: since the order of operations says that order does not matter with multiplication and division but does matter between multiplication and addition/subtraction, or multiplication and exponents, choose a function that has only multiplication and/or division. Only answer choice (B) qualifies.
The question is asking, “For which function is performing the function on x and THEN multiplying by \(\frac{1}{2}\) the equivalent of performing the function on \(\frac{1}{2}\) of x?”
The fastest method is to use logic: since the order of operations says that order does not matter with multiplication and division but does matter between multiplication and addition/subtraction, or multiplication and exponents, choose a function that has only multiplication and/or division. Only answer choice (B) qualifies.
