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# If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms

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If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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24 Aug 2012, 05:04
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If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(\frac{x}{5})$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$
[Reveal] Spoiler: OA

Last edited by Bunuel on 18 Jul 2013, 22:26, edited 2 times in total.
Edited the question.

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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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24 Aug 2012, 05:58
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manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$\frac{f(5x)}{f(x/5)}$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

Say $$x=1$$, then:

$$f(x) = f(1)=\frac{125}{1^{3}}=125$$;

$$f(5x)=f(5)=\frac{125}{5^3}=1$$;

$$f(\frac{x}{5})=f(\frac{1}{5})=\frac{125}{(\frac{1}{5})^3}=125*125$$.

$${f(5x)}*{f(\frac{x}{5})}={f(5)}*{f(\frac{1}{5})}=125^2$$ and since $$f(x) = f(1)=125$$, then $${f(5x)}*{f(\frac{x}{5})}=125^2=(f(x))^2$$.

Hope it's clear.
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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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24 Aug 2012, 06:05
Bunuel wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

Thanks Bunuel.

The question really troubled me.
I tried methods and still could not arrive at correct answer.
Your verdict has confirmed my doubt.

I shall edit the question.

PS: You already did that. Thanks again.

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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03 Jul 2013, 01:23
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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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03 Jul 2013, 08:44
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manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

lets 125 =A
so we can write f(x)=A/(X^3)...........(1
Now lets sove

f(5x)*f(x/5)=(A/(5X)^3)*(A/(X/5)^3)====>(A^2/X^6)===>(A/X^3)^2==(F(X))^2==>USING(1)

Hence A
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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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04 Jul 2013, 00:48
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Expert's post
manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

1. $$f(5x) = 125/125*x^3 = 1/x^3$$
2.$$f(x/5) = 125 * 125/x^3 = 125^2 / x^3$$
3. $$f(5x) * f(x/5) = 125 ^ 2/ x^6 = (f(x)) ^ {2}$$
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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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18 Jul 2013, 18:30
SravnaTestPrep wrote:
manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

1. $$f(5x) = 125/125*x^3 = 1/x^3$$
2.$$f(x/5) = 125 * 125/x^3 = 125^2 / x^3$$
3. $$f(5x) * f(x/5) = 125 ^ 2/ x^6 = (f(x)) ^ {2}$$

In step 3, you have multiplied 1. & 2. What would be the answer if you divide 1. by 2. ? I am kinda confused. I get f(5x)/f(x/5) = 1/5^6. Help, thanks!

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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18 Jul 2013, 22:26
mneeti wrote:
SravnaTestPrep wrote:
manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

1. $$f(5x) = 125/125*x^3 = 1/x^3$$
2.$$f(x/5) = 125 * 125/x^3 = 125^2 / x^3$$
3. $$f(5x) * f(x/5) = 125 ^ 2/ x^6 = (f(x)) ^ {2}$$

In step 3, you have multiplied 1. & 2. What would be the answer if you divide 1. by 2. ? I am kinda confused. I get f(5x)/f(x/5) = 1/5^6. Help, thanks!

The questions is "what is the value of $$f(5x)*f(\frac{x}{5})$$..." (f(5x) multiplied by f(x/5)) not "what is the value of $$\frac{f(5x)}{f(\frac{x}{5})}$$..." (f(5x) divided by f(x/5)).
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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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18 Jul 2013, 22:47
Bunuel wrote:
mneeti wrote:

The questions is "what is the value of $$f(5x)*f(\frac{x}{5})$$..." (f(5x) multiplied by f(x/5)) not "what is the value of $$\frac{f(5x)}{f(\frac{x}{5})}$$..." (f(5x) divided by f(x/5)).

Please refer your Ist response in this topic, the quoted part asks to find $$\frac{f(5x)}{f(\frac{x}{5})}$$. Also, the question I have from my source of practice questions requires to find the $$\frac{f(5x)}{f(\frac{x}{5})}$$. Thanks!

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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18 Jul 2013, 22:49
mneeti wrote:
Bunuel wrote:
mneeti wrote:

The questions is "what is the value of $$f(5x)*f(\frac{x}{5})$$..." (f(5x) multiplied by f(x/5)) not "what is the value of $$\frac{f(5x)}{f(\frac{x}{5})}$$..." (f(5x) divided by f(x/5)).

Please refer your Ist response in this topic, the quoted part asks to find $$\frac{f(5x)}{f(\frac{x}{5})}$$. Also, the question I have from my source of practice questions requires to find the $$\frac{f(5x)}{f(\frac{x}{5})}$$. Thanks!

Please refer to the same post: in order the answer to be A, the question should read:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(\frac{x}{5})$$ in terms of f(x)?
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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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24 Aug 2013, 06:09
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Bunuel wrote:
manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$\frac{f(5x)}{f(x/5)}$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

Say $$x=1$$, then:

$$f(x) = f(1)=\frac{125}{1^{3}}=125$$;

$$f(5x)=f(5)=\frac{125}{5^3}=1$$;

$$f(\frac{x}{5})=f(\frac{1}{5})=\frac{125}{(\frac{1}{5})^3}=125*125$$.

$${f(5x)}*{f(\frac{x}{5})}={f(5)}*{f(\frac{1}{5})}=125^2$$ and since $$f(x) = f(1)=125$$, then $${f(5x)}*{f(\frac{x}{5})}=125^2=(f(x))^2$$.

Hope it's clear.

Can we take constant 5 outside of f(5x) i.e 5f(x) ?
I was able to arrive at the answer by taking constant outside of the function,but not sure whether it is correct approach or not.

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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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25 Aug 2013, 06:41
abid1986 wrote:
Bunuel wrote:
manulath wrote:
If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$\frac{f(5x)}{f(x/5)}$$ in terms of f(x)?

A. $$(f(x))^{2}$$

B. $$f(x^{2})$$

C. $$(f(x))^{3}$$

D. $$f(x^{3})$$

E. $$f(125x)$$

If $$f(x) = \frac{125}{x^{3}}$$, what is the value of $$f(5x)*f(x/5)$$ in terms of f(x)?

Say $$x=1$$, then:

$$f(x) = f(1)=\frac{125}{1^{3}}=125$$;

$$f(5x)=f(5)=\frac{125}{5^3}=1$$;

$$f(\frac{x}{5})=f(\frac{1}{5})=\frac{125}{(\frac{1}{5})^3}=125*125$$.

$${f(5x)}*{f(\frac{x}{5})}={f(5)}*{f(\frac{1}{5})}=125^2$$ and since $$f(x) = f(1)=125$$, then $${f(5x)}*{f(\frac{x}{5})}=125^2=(f(x))^2$$.

Hope it's clear.

Can we take constant 5 outside of f(5x) i.e 5f(x) ?
I was able to arrive at the answer by taking constant outside of the function,but not sure whether it is correct approach or not.

It depends on HOW are you doing that. Please show your work.
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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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25 Aug 2013, 11:21
f(5x) = 5f(x)
f(x/5)= 1/5f(x)
Then;
f(5x)*f(x/5) = 5f(x)*1/5f(x) =
$$(f(x))^2$$

Is this approach fine?

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Re: If f(x)=125/x^3, what is the value of f(5x)/f(x/5) in terms [#permalink]

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25 Aug 2013, 11:36
abid1986 wrote:
f(5x) = 5f(x)
f(x/5)= 1/5f(x)
Then;
f(5x)*f(x/5) = 5f(x)*1/5f(x) =
$$(f(x))^2$$

Is this approach fine?

No, that's not correct.
$$f(x) = \frac{125}{x^{3}}$$,
$$f(5x) = \frac{125}{(5x)^{3}}=\frac{1}{x^3}$$.
$$f(\frac{x}{5}) = \frac{125}{(\frac{x}{5})^{3}}=\frac{125^2}{x^3}$$.

So, $$f(x)=125*f(5x)$$ and $$f(\frac{x}{5})=125*f(x)$$

Hope it helps.
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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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30 Aug 2014, 08:13
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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02 Sep 2014, 03:31
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$$f(5x) * f(\frac{x}{5}) = \frac{125}{(5x)^3} * \frac{125}{(\frac{x}{5})^3}= \frac{125}{x^3} * \frac{125}{x^3} = [f(x)]^2$$

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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24 Jan 2017, 07:22
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms [#permalink]

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18 Jul 2017, 19:29
This can be solved like this. It is important to understand the difference between (a) and (b) for instance (F(x))^2 means you square the whole function while F(x^2) means you plug in X^2 everytime you see X. A good video explaining how function notation works is here. It's not my video but he explains it well if you missed this step.

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Re: If f(x)=125/x^3, what is the value of f(5x)*f(x/5) in terms   [#permalink] 18 Jul 2017, 19:29
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