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

Question

(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

Marcab · Answer

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.

Archit143 · Answer

thanks marcab just wanted to be sure of my approach

pacifist85 · Answer

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

ranaazad · Answer

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.

chetan2u · Answer

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..

adiagr · Answer

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

A is the answer

JeffTargetTestPrep · Answer

Since (x + 7) is a factor of x^2 - mx - 56, we know that x = -7 will satisfy the equation x^2 - mx - 56 = 0; therefore, we can substitute -7 for x and we have:
 
(-7)^2 - 7m - 56 = 0
 
49 - 7m = 56
 
-7m = -7
 
m = 1
 
Answer: A

arvind910619 · Answer

Imo A
Substitute the value x=-7
We get m=1

Sent from my ONE E1003 using  GMAT Club Forum mobile app

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

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

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

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

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards