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alrussell
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In the GMAT Math Bible, in the first few pages it gives the Updated on: 22 Nov 2007, 04:22
Originally posted by alrussell on 22 Nov 2007, 04:10.
Last edited by alrussell on 22 Nov 2007, 04:22, edited 1 time in total.
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In the GMAT Math Bible, in the first few pages it gives the following as an e.g. DS question:

Is x positive

1) x^2 = 16
2) x^2 – 6x +8 = 0

Clearly A is insuff. As it could be 4/-4

It then goes on to say that 2) gives us x =-2 / -4. I don’t understand this… I factor it out to (x – 2)(x – 4).
If that’s the case, then surely x = 2 / 4?

What am I missing here!!??



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bmwhype2
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In the GMAT Math Bible, in the first few pages it gives the 22 Nov 2007, 04:17
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alrussell
In the GMAT Math Bible, in the first few pages it gives the following as an e.g. DS question:

Is x positive

1) x2 = 16
2) x2 – 6x +8 = 0

Clearly A is insuff. As it could be 4/-4

It then goes on to say that 2) gives us x =-2 / -4. I don’t understand this… I factor it out to (x – 2)(x – 4).
If that’s the case, then surely x = 2 / 4?

What am I missing here!!??
Show more


1. even exponents hide the sign of the base. X^2 can be -4 or 4.

2. (x-2)(x-4)
x=2 and X=4
Both positive

answer is B
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alrussell
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In the GMAT Math Bible, in the first few pages it gives the 22 Nov 2007, 04:23
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bmwhype2
alrussell
In the GMAT Math Bible, in the first few pages it gives the following as an e.g. DS question:

Is x positive

1) x2 = 16
2) x2 – 6x +8 = 0

Clearly A is insuff. As it could be 4/-4

It then goes on to say that 2) gives us x =-2 / -4. I don’t understand this… I factor it out to (x – 2)(x – 4).
If that’s the case, then surely x = 2 / 4?

What am I missing here!!??

1. even exponents hide the sign of the base. X^2 can be -4 or 4.

2. (x-2)(x-4)
x=2 and X=4
Both positive

answer is B
Show more


I understand that, but my question is, for statement 2 - why has it told me that the options are x =-2 / x = -4? Surely it's forgotten to switch the signs?
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tarek99
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In the GMAT Math Bible, in the first few pages it gives the 22 Nov 2007, 05:15
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The only issue I see you have here is an issue of vocabulary. I'll explain:

(x – 2)(x – 4)

2 or 4 This is called solution or roots

-2 or -4 are factors.

so when a question or an explanation tells you that -2 is a factor, you write it as (x-2). But if it tells you that -2 is a solution or a root, you write it as (X+2)

hope that's clear
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alrussell
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In the GMAT Math Bible, in the first few pages it gives the 22 Nov 2007, 06:13
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tarek99
The only issue I see you have here is an issue of vocabulary. I'll explain:

(x – 2)(x – 4)

2 or 4 This is called solution or roots

-2 or -4 are factors.

so when a question or an explanation tells you that -2 is a factor, you write it as (x-2). But if it tells you that -2 is a solution or a root, you write it as (X+2)

hope that's clear
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Thanks. But THe book simply says that from the equation x is -2 / -4... it doesnt employt the terminology you have ... is there a reason they would have solved for x as such?
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asdert
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In the GMAT Math Bible, in the first few pages it gives the 24 Nov 2007, 14:58
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(x-4)(x-2)=0

This means either x-4 = 0 or x-2 = 0

Solve for both cases:

(x-4)(0)=0
x=+4

(x-2)(0)=0
x=+2

So x either 2 or 4, boh positive, so B is suff.
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alrussell
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In the GMAT Math Bible, in the first few pages it gives the 24 Nov 2007, 17:15
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asdert
(x-4)(x-2)=0

This means either x-4 = 0 or x-2 = 0

Solve for both cases:

(x-4)(0)=0
x=+4

(x-2)(0)=0
x=+2

So x either 2 or 4, boh positive, so B is suff.
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That's my point. The math bible ... which has come recommended to me on here (though it does seem good otherwise) says essentially "thus x = -4 or -2" ... I think he's forgotten to switch the signs.. but it threw me ...
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spider
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In the GMAT Math Bible, in the first few pages it gives the 24 Nov 2007, 17:23
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alrussell
In the GMAT Math Bible, in the first few pages it gives the following as an e.g. DS question:

Is x positive

1) x^2 = 16
2) x^2 – 6x +8 = 0

Clearly A is insuff. As it could be 4/-4

It then goes on to say that 2) gives us x =-2 / -4. I don’t understand this… I factor it out to (x – 2)(x – 4).
If that’s the case, then surely x = 2 / 4?

What am I missing here!!??
Show more


B for sure..since the values for x from the 2nd equation are 2 or 4 which are both +ive



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!