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solving for new equation -x-5<4 => x>-9 x+5<4 => x< -1

This means x lies between -9 and -1 or -9<x<-1 hence it doesn't satisfy the equation 1<x<9

C) |x-1|<9

Negative: (x-1)<0 => x>1 This should be x<1 Positive: (x-1)>0 => x>1

solving for new equation -x+1<9 => x>-8 x-1<9 => x<8 This should be x<10

it means x lies between -8 and 8 or -8<x<8 x between -8 & 10 it doesn't satisfy the equation 1<x<9

D)|-5+x|<4

Negative: (-5+x)<0 => x>5 This should be x<5 Positive: (-5+x) >0 => x>5

solving for new equation -5+x<4 => 5-x<4 => x>1 This is the case x<5, your final solution is correct, x>1 -5+x<4 => x<9 This means x>1 but less than 9 or 1<x<9 Hence it proved the relation.

Re: Math: Absolute value (Modulus) [#permalink]
29 Nov 2010, 21:04

Let’s consider following examples,

Example #1I am not understanding this example and really struggling with modulus? Can someone please elaborate and explain in further detail? From this post I can't see how I would use this on every modulus problem? Q.:|x+3| - |4-x| = |8+x|. How many solutions does the equation have? Solution: There are 3 key points here: -8, -3, 4. So we have 4 conditions:

a) x < -8. -(x+3) - (4-x) how did we get -(x+3) here?= -(8+x) --> x = -1. We reject the solution because our condition is not satisfied (-1 is not less than -8)

b) -8 \leq x < -3. -(x+3) - (4-x) = (8+x) --> x = -15. We reject the solution because our condition is not satisfied (-15 is not within (-8,-3) interval.)

c) -3 \leq x < 4. (x+3) - (4-x) = (8+x) --> x = 9. We reject the solution because our condition is not satisfied (-15 is not within (-3,4) interval.)

d) x \geq 4. (x+3) + (4-x) = (8+x) --> x = -1. We reject the solution because our condition is not satisfied (-1 is not more than 4)

I am totally lost with this post and also with other modulus problems I looked up in Gmat club thank you very much for your help in advance!!!!!

Re: Math: Absolute value (Modulus) [#permalink]
30 Nov 2010, 06:09

1

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Expert's post

a) x < -8. -(x+3) - (4-x) = -(8+x) --> x = -1. We reject the solution because our condition is not satisfied (-1 is not less than -8)

if x < -8, (x + 3) is always negative. So, modulus is non-negative and we need to change a sign: |x+3| = - (x+3) for x<-8 For example, if x = -10, |-10+3| = |-7| = 7 -(-10+3) = -(-7) = 7

In other words, |x| = x if x is positive and |x|=-x if x is negative.

Re: Math: Absolute value (Modulus) [#permalink]
01 Dec 2010, 10:49

walker wrote:

a) x < -8. -(x+3) - (4-x) = -(8+x) --> x = -1. We reject the solution because our condition is not satisfied (-1 is not less than -8)

if x < -8, (x + 3) is always negative. So, modulus is non-negative and we need to change a sign: |x+3| = - (x+3) for x<-8 For example, if x = -10, |-10+3| = |-7| = 7 -(-10+3) = -(-7) = 7

In other words, |x| = x if x is positive and |x|=-x if x is negative.

Thanks for the explanation Walker. So would this then be correct:

Example #1 Q.: |x+3| - |4-x| = |8+x|. How many solutions does the equation have? Solution: There are 3 key points here: -8, -3, 4. So we have 4 conditions:

a) x < -8. -(x+3) - (4-x) = -(8+x) --> x = -1. |x+3|is always - because x<-8 but l4-xl will always be positive because 4-x= a +, l8+xl will also be negative as well given x<-8

b) -8 \leq x < -3. -(x+3) - (4-x) = (8+x) --> x = -15. on this one lx+3l again can only be negative and l4-xl will be still always be negative but given the range l8+xl may only be postiive hence l8+xl has to be positive or non-negative (o)?

c) -3 \leq x < 4. (x+3) - (4-x) = (8+x) --> x = 9. so on this term first one is positive second term continues to be positive and 3rd term is positive.

d) x \geq 4. (x+3) + (4-x) = (8+x) --> x = -1. on this one first term obviously positive second term is negative therefore the sign changes from - to + so from -(4-x) to +(4-x) since this will always be negative for given x range? And a negative times a negative gets us to a positive?

I just want to make sure I am thinking about this correctly. Thanks walker!

Re: Math: Absolute value (Modulus) [#permalink]
01 Dec 2010, 11:19

Sorry walker one more question on the below as I review this Thinking of inequality with modulus as a segment at the number line.

For example, Problem: 1<x<9. What inequality represents this condition? Image A. |x|<3 B. |x+5|<4 C. |x-1|<9 D. |-5+x|<4 E. |3+x|<5 Solution: 10sec. Traditional 3-steps method is too time-consume technique. First of all we find length (9-1)=8 and center (1+8/2=5) of the segment represented by 1<x<9. Now, let’s look at our options. Only B and D has 8/2=4 on the right side and D had left site 0 at x=5. Therefore, answer is D.

Why would be looking for 4 on the right side? shouldn't this be 5 as the midpoint? And why do we want the left side to be 0 at x=5? Id like to understand this a little better

II. Converting inequalities with modulus into range expression. In many cases, especially in DS problems, it helps avoid silly mistakes.

For example, |x|<5 is equal to x e (-5,5). |x+3|>3 is equal to x e (-inf,-6)&(0,+inf)

How did you convert lx+3l>3 into (-inf,-6) and (0,+inf)

Sorry these maybe simple questions but I just want to grasp the concept firmly

Re: Math: Absolute value (Modulus) [#permalink]
01 Dec 2010, 13:21

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Expert's post

A couple of figures to see what modulus means.

It is quite convenient to remember the definition of modulus. |x| is the distance of x from 0 on the number line. So if |x| = 1, we are looking for points which are at a distance 1 away from 0. If |x| < 1, we are looking for points which are at a distance less than 1 away from 0. If |x| > 1, we are looking for points which are at a distance more than 1 away from 0.

Attachment:

Ques1.jpg [ 14.57 KiB | Viewed 11699 times ]

If |x - 5| < 4, now we are looking for points at a distance less than 4 away from 5.

Re: Math: Absolute value (Modulus) [#permalink]
01 Dec 2010, 18:35

Great thanks for the explanation on that Karishma - very very very helpful.

Can you also help me with how we converted lx+3l>3 into (-inf,-6) and (0,+inf)?

Is this because x+3>=0 (non-negative) and negative x+3<0 (negative). So we get to x+3=3 so x=0 which fits condition 1 above and this will go to infinity

and then

-(x+3)=3 so you get -x=6 or x=-6 which fits condition two above so x<-6 so therefore it goes to negative infinity?

Re: Math: Absolute value (Modulus) [#permalink]
01 Dec 2010, 19:28

3

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Expert's post

gettinit wrote:

Great thanks for the explanation on that Karishma - very very very helpful.

Can you also help me with how we converted lx+3l>3 into (-inf,-6) and (0,+inf)?

Is this because x+3>=0 (non-negative) and negative x+3<0 (negative). So we get to x+3=3 so x=0 which fits condition 1 above and this will go to infinity

and then

-(x+3)=3 so you get -x=6 or x=-6 which fits condition two above so x<-6 so therefore it goes to negative infinity?

I think I am close. appreciate the help.

Yes, lx+3l>3 gives us two cases

Case 1: x + 3 >= 0 or x > = -3 Then (x + 3) > 3 or we can say x > 0

Case 2: x + 3 < 0 or x < -3 Then -(x + 3) > 3 or we can say x < -6

So either x > 0 which translates to (0, inf) or x < -6 which translates to (-inf, -6)

OR consider that lx+3l>3 means distance of x from -3 is more than 3. If you go to 3 steps to right from -3, you reach 0. Anything after than is ok. If you go 3 steps to left from -3, you reach -6. Anything to its left is ok.