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Quadratic Equation question

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by gxj007 » Sun Dec 14, 2008 11:28 am
How do you solve a quadratic equation if there are not numbers which multiply to the last number and add to the middle number?

ie:

x^2 - 2x - 8 = 0

or x^2 - 2x - 7 =0

I know that if you have someting like x^2 - 2x - 3 = 0 you can factor it to
x-3 and x+1 but how about the cases above?

thanks!
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Source: — Quantitative Reasoning |

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by Brent@GMATPrepNow » Sun Dec 14, 2008 3:19 pm
The "official" answer would be to use the quadratic formula, which is far too awful to try to type here. Google "quadratic formula" to learn more.

GMAC includes the quadratic formula in its curriculum but, to date, I have not seen one single official GMAT question that requires the quadratic formula. I do know one instructor who says a former student claims to have had a question that required the quadratic formula, but I'll believe it when I see it. If you ask me, it sounds like an urban myth :D

The whole quadratic formula thing is a bit of a paradox in that, if you were to see a question requiring this formula, then you are probably scoring in the 750+ range, in which case you probably already know the formula. If you are not scoring in the 750+ range, then you won't see questions requiring the formula so don't waste your time memorizing it. Of course, all of this assumes that there are such questions.

Are there any other GMAT instructors out there who would like to weigh in on this? Am I missing anything?
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Re: Quadratic Equation question

by logitech » Sun Dec 14, 2008 4:03 pm
gxj007 wrote:How do you solve a quadratic equation if there are not numbers which multiply to the last number and add to the middle number?

ie:

x^2 - 2x - 8 = 0

or x^2 - 2x - 7 =0

I know that if you have someting like x^2 - 2x - 3 = 0 you can factor it to
x-3 and x+1 but how about the cases above?

thanks!
for example

x^2 - 2x - 8 = 0

AX^2+BX+C=0

easy method

you have to find the factors of both A and C and try to get B as below:

A = A1xA2
C=C1xC2

And B must be equal to:

B= A1xC2 + A2XC1

lets apply this to:

x^2 - 2x - 8 = 0

1x -4
1x +2

(2x + (-4x) = -2x ( remember B =-2)

so

x^2 - 2x - 8 = 0

(x-4)(x+2)=0
LGTCH
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by Brent@GMATPrepNow » Sun Dec 14, 2008 4:20 pm
Oops, I didn't even notice that the first equation (x^2 - 2x - 8 = 0 ) is factorable.

What about the second equation (x^2 - 2x - 7 =0) ?
Any thoughts about the quadtatic formula?

Do any instructors out there delve deeply into this formula?

I'm quite curious.
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by logitech » Sun Dec 14, 2008 4:31 pm
Brent Hanneson wrote:Oops, I didn't even notice that the first equation (x^2 - 2x - 8 = 0 ) is factorable.

What about the second equation (x^2 - 2x - 7 =0) ?
Any thoughts about the quadtatic formula?

Do any instructors out there delve deeply into this formula?

I'm quite curious.
The Quadratic Formula is out of GMAT's scope BUT it does not hurt to learn it..
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LGTCH
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by gxj007 » Mon Dec 15, 2008 9:36 am
Thanks for your responses. So is it safe to say that the strategy for completing the square is probably out of scope as well? I guess I just want to ensure that I'm not wasting time going too far in depth with certain concepts if they won't even show up on the test.

thanks again!
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by logitech » Mon Dec 15, 2008 10:06 am
gxj007 wrote:Thanks for your responses. So is it safe to say that the strategy for completing the square is probably out of scope as well? I guess I just want to ensure that I'm not wasting time going too far in depth with certain concepts if they won't even show up on the test.

thanks again!
Old greek saying:

Eat the grape, and don't ask where it s coming from.

Just learn the formula and use it when you need it.

Good luck!
LGTCH
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