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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] New post 05 Dec 2012, 07:00
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69% (01:46) correct 31% (00:56) wrong based on 108 sessions
(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, 07:03, 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] New post 05 Dec 2012, 10:01
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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] New post 05 Dec 2012, 23:20
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] New post 24 Oct 2014, 19:37
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Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m [#permalink] New post 15 Jan 2015, 02:53
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
Re: (x + 7) is a factor in x^2 - mx - 56. What is the value of m   [#permalink] 15 Jan 2015, 02:53
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