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(x + 7) is a factor in x^2 - mx - 56. What is the value of m

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(x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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05 Dec 2012, 08:00
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(x + 7) is a factor in x^2 - mx - 56. What is the value of m?

A. 1
B. 8
C. 14
D. 15
E. 16
[Reveal] Spoiler: OA

Last edited by Bunuel on 05 Dec 2012, 08:03, edited 1 time in total.
Edited the question.
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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05 Dec 2012, 11:01
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Given the fact that $$(x+7)$$ is the factor of the equation,
so it claerly means that one value of x=-7; the other we don't know and we don't have to.

The equation can be written in factor form as:
$$(x+7)*(x+a)$$ where a is the other value of x.

On putting the value of x=-7 in the equation,
$$x^2-mx-56=0$$

we get:

$$49+7m-56=0$$

$$-7=-7m$$
$$m=1$$

+1 A.
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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06 Dec 2012, 00:20
thanks marcab just wanted to be sure of my approach
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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24 Oct 2014, 20:37
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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: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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15 Jan 2015, 03:53
I solved the second degree equation and found it like this:

x^2 - mx - 56 = 0
(x-8)(x+7) = 0
x=8 or x= -7

Substituting both values for x in the equation we find:
x^2 - mx - 56
=> (-7)^2 - m(-7) - 56
=> 49 - 56 + 7m
=> -49 + 56 = 7m
=> 7 = 7m
=> m = 1

And with 8, using a similar process we end up with:
64-56=8m
8=8m
m=1

Ao, ANS A
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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28 May 2016, 23:37
l got the same answer. lt's not stated in the question stem that x^2 - mx - 56 = 0. Based on what information do we know (x^2 - mx - 56) equals to zero? Could someone please clarify?

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Curious mind.
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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29 May 2016, 00:14
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l got the same answer. lt's not stated in the question stem that x^2 - mx - 56 = 0. Based on what information do we know (x^2 - mx - 56) equals to zero? Could someone please clarify?

Regards,
Curious mind.

Hi,

x^2-mx-56 can be any value depending on value of x.....
this equation is that of PARABOLA, so you will get a parabola when you draw a graph of it...

Just remember for GMAT that
but when we factor or find roots, we work for ONLY those values of x at which the equation gives us a 0 value..

That is why when we are factoring the equation here we take the equation to be equal to 0...

so If I tell you the speed of something is dependent on x^2-15x+56
and then ask you speed at say 9h second, substitute x as 9 and find the answer..
But if I ask you at what values will the speed be 0, put the equation as 0 and find values of x..
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink]

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29 May 2016, 00:44
Archit143 wrote:
(x + 7) is a factor in x^2 - mx - 56. What is the value of m?

A. 1
B. 8
C. 14
D. 15
E. 16

If (x+7) is a factor of expression x^2 - mx - 56, then if we substitute x = -7, in the given expression, the value of expression will be zero.

So, (-7)^2 - (m)(-7) - 56 = 0

49+7m-56 = 0

7m-7=0

m=1

Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m   [#permalink] 29 May 2016, 00:44
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