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# For which of the following functions is g(c-d) = g(c) - g(d)

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For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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11 Feb 2013, 10:09
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For which of the following functions is g(c-d) = g(c) - g(d) for all positive numbers c and d?

A g(x) = x^3
B g(x) = x+5
C g(x) = $$\sqrt{3x}$$
D g(x) = 5x
E g(x) 15/x

Sometimes I read a problem 5 times in a row... and I still cannot understand what the question is asking. This was one of those...
[Reveal] Spoiler: OA
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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11 Feb 2013, 10:18
5
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exploringm wrote:
For which of the following functions is g(c-d) = g(c) - g(d) for all positive numbers c and d?
A g(x) = x^3
B g(x) = x+5
C g(x) = $$\sqrt{3x}$$
D g(x) = 5x
E g(x) 15/x

Sometimes I read a problem 5 times in a row... and I still cannot understand what the question is asking. This was one of those...

This problem arises because in such type of questions the original function is not given. Instead, we have to find a function which can satisfy the constraint mentioned in the question.
Constraint--> g(c-d) = g(c) - g(d)--->(1), where (c-d), c, d are the values of x.
I am directly putting the value in the answer. You can verify the incorrect option by yourself.

Function as per option D is --->g(x) = 5x
Put the value of x = (c-d), we get --->g(c-d) = 5 (c-d) = 5c - 5d----(LHS)
Put the value of x = c, we get --->g(c) = 5 (c) = 5c ----(RHS)
Put the value of x = d, we get --->g(d) = 5 (d) = 5d----(RHS)

Equate RHS & LHS we get--> 5c - 5d = 5c - 5d
Thus equal.

Emma
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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16 Feb 2013, 13:21
In such questions these are the steps I find useful!!

1) The first step is to check whether you could solve the expression stated in the question( in case the expression is too long) or substitute the expression into the solutions in some manner so as to solve it quickly. If so, this is the quickest way out and usually with a little bit of practice, you can start judging whether this step would work or not before you place your pen on the paper to solve the question.

2) The next method is to find whether the solutions can be substituted into the original expression in some way that is fast and quick. This usually takes some time and hence, mostly does not work. However, if the answer options involve numbers instead of expressions, this might work better.

This method is only for those questions in which the answer options and the questions both contain expressions.

3) You could try substitution of base values to confirm.

.
exploringm wrote:
For which of the following functions is g(c-d) = g(c) - g(d) for all positive numbers c and d?

A g(x) = x^3
B g(x) = x+5
C g(x) = $$\sqrt{3x}$$
D g(x) = 5x
E g(x) 15/x

Sometimes I read a problem 5 times in a row... and I still cannot understand what the question is asking. This was one of those...
Intern
Joined: 20 Apr 2013
Posts: 24
Concentration: Finance, Finance
GMAT Date: 06-03-2013
GPA: 3.3
WE: Accounting (Accounting)
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Kudos [?]: 2 [0], given: 99

Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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30 Apr 2013, 22:54
Why is option C Wrong
\sqrt{3(c-d)} = \sqrt{(3c)}-\sqrt{(3d)}
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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30 Apr 2013, 23:08
Rajkiranmareedu wrote:
Why is option C Wrong
\sqrt{3(c-d)} = \sqrt{(3c)}-\sqrt{(3d)}

Option C says g(x) = $$\sqrt{3x}$$

So, g(c-d) = $$\sqrt{c-d}$$
g(c) = $$\sqrt{c}$$
g(d) = $$\sqrt{d}$$

We cannot say that $$\sqrt{c-d}$$ = $$\sqrt{c}$$ - $$\sqrt{d}$$ for all values of c and d
So we cannot say that g(c-d) = g(c) - g(d) for all values of c and d.

Thus, option c is wrong.
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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04 Jul 2013, 19:48
exploringm wrote:
For which of the following functions is g(c-d) = g(c) - g(d) for all positive numbers c and d?

A g(x) = x^3
B g(x) = x+5
C g(x) = $$\sqrt{3x}$$
D g(x) = 5x
E g(x) 15/x

Sometimes I read a problem 5 times in a row... and I still cannot understand what the question is asking. This was one of those...

put c = 3 and d = 2 or directly substitute c and d

we get option D,
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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14 Jan 2015, 01:30
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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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14 Jan 2015, 23:50
3
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Expert's post
Hi All,

In these types of "function" questions, you can TEST VALUES (especially really simple ones) to find the correct answer.

Let's TEST...
C = 2
D = 1

The question asks us to find the equation that gives us the same result when...

g(2-1) = g(1) is calculated

and when

g(2) - g(1) is calculated

g(1) = 1^3 = 1
g(2) - g(1) = 2^3 - 1^3 = 8 - 1 = 7
1 does NOT = 7

g(1) = 1+5 = 6
g(2) - g(1) = (2+5) - (1+5) = 7 - 6 = 1
6 does NOT = 1

g(1) = \sqrt{3}
g(2) - g(1) = \sqrt{6} - \sqrt{3} = \sqrt{3}(\sqrt{2} - 1)
\sqrt{3} does NOT = \sqrt{3}(\sqrt{2} - 1)

g(1) = 5(1) = 5
g(2) - g(1) = 5(2) - 5(1) = 10 - 5 = 5
5 DOES = 5

g(1) = 15/1 = 15
g(2) - g(1) = 15/2 - 15/1 = -15/2
15 does NOT = -15/2

[Reveal] Spoiler:
D

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For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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21 Jan 2015, 20:59
A g(x) = x^3

Power of "x" is changed, so IGNORE

B g(x) = x+5

"x" is added by constant, so IGNORE

C g(x) = \sqrt{3x}

Power of "x" is changed, so IGNORE

D g(x) = 5x

"x" is multiplied by constant 5, so required result is true

E g(x) 15/x

$$\frac{15}{x} = 15 * x^{-1}$$

Again power of "x" is changed, so IGNORE

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Re: For which of the following functions is g(c-d) = g(c) - g(d) [#permalink]

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20 Jun 2016, 03:32
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: For which of the following functions is g(c-d) = g(c) - g(d)   [#permalink] 20 Jun 2016, 03:32
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