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File comment: |x+2|=3-2x
docu1.png [ 43.31 KiB | Viewed 166238 times ]
I had a PM and a profile comment asking about the absolute modulus, its concept and in particular a Question discussed on various occassion " How many solutions does the equation |x+3|-|4-x|=|8+x| have?.... docu1.png [ 43.31 KiB | Viewed 166238 times ]
Just thought to write down few concepts I have gathered. I have not gone through various Topics on Absolute Modulus in this Forum, so maybe few points are repetition.
Although difficult for a topic like this, I'll try to follow KISS- Keep It Short and Simple. So, let me touch the concepts now..
what is absolute modulus?
Absolute modulus is the numeric value of any number without any sign or in other words ' the distance from the origin'. It will always be positive.
What kind of Qs can one see in GMAT?
The Q will ask either the values of x or how many values can x take?..
most often what one can encounter is a linear Equation with...
a) only one Mod
eg.. |x+2| + 2x= 3..
b) two mods..
|x+2|=|x-3|+1..
c) three mods.. very rare
|x+3|-|4-x|=|8+x| ..
What are the methods
..three methods..
1) As the property suggests, Open each modulus in both +ive and -ive ....
2) Critical value
3) Graphical method..
Opening each modulus
It is a time consuming process where you open each mod in both positive and negative and the number of Equations thus formed will increase as we increase the no of mods..
a) only one Mod
eg.. |x+2| + 2x= 3..
i) (x+2) + 2x=3.. 3x+2=3 x=1/3.. valid value
ii) -(x+2)+2x=3.. x-2=3..x=5...
but if we substitute x=5 in |x+2| + 2x= 3..... (x+2) will turn out to be a positive value, while we took x=2 to be negative so discard
so one value of x..
b) two mods..
|x+2|=|x-3|+1..
here you will have four equations..
i)(x+2)=(x-3)+1.. both positive
ii)-(x+2)=-(x-3)+1.. both negative
iii)-(x+2)=(x-3)+1..one positive and other negative
iv)(x+2)=-(x-3)+1.. opposite of the one on top
c) three mods.. very rare
|x+3|-|4-x|=|8+x| ..
it will further increase the number of equations..
Suggestion.. time consuming and susceptible to errors in opening of brackets and at times requires more to negate the values found as in first example.
Critical method
lets find what happens in this before trying the Qs as this was the main query..
Step 1 :- for each mod, there is a value of x that will make that mod to 0..
Step 2 :- the minimum value of a mod will be 0 and at this value of x, the mod has the lowest value...
Once we know this critical value, we work on the mod for values lesser than(<) that or more than(>)that and including the critical value in either of them,
we assign a sign, + or -, depending on what will happen to the value inside the mod in each scenario(in one scenario it will be positive and in other, negative)..
Step 3 :- after assigning the sign, we solve for x and see if the value of x that we get is possible depending on which side of critical value we are working on..
So what are we doing here
We are assuming a certain region for value of x and then solving for x.. If the value found matches the initial assumption, we take that as a solution or discard that value, which would mean that there is no value of x in that assumed region
lets see the three examples
a) only one Mod
eg.. |x+2| + 2x= 3..
here x+2 will be 0 at x=-2..
so Critical value =-2..
so two regions are <-2 and >= -2
i)when x<-2, |x+2|will be given negative sign.. for this assign any value in that region say -3 in this case x+2 will become -3+2=-1 hence a negative sign..
-(x+2)+2x=3..
x-2=3.. x=5, which is not in the region <-2.. so not a valid value..
ii)when x>=-2, |x+2|will be given positive sign.. for this assign any value in that region say 3 in this case x+2 will become 3+2= 5 hence a positive sign..
(x+2)+2x=3..
3x+2=3.. x=1/3, which is in the region >=-2.. so a valid value..
b) two mods..
|x+2|=|x-3|+1..
critical values -2 and 3...
so regions are <-2, -2<=x<3, x>=3..
i) x<-2...
x+2 will be -ive and x-3 will be negative ..
eq becomes -(x+2)=-(x-3)+1.. both negative
-x-2=-x+3+1..... no values..
ii) \(-2<=x<3\)..
x+2 will be positive and x-3 will be negative ..
eq becomes (x+2)=-(x-3)+1..
x+2=-x+3+1..
x=1.. valid value
iii)x>=3..
x+2 will be positive and x-3 will be positive ..
eq becomes (x+2)=(x-3)+1..
x+2=x-3+1..
no valid value..
so the solution is x=1
c) three mods.. very rare
|x+3|-|4-x|=|8+x| ..
its time consuming and can be solved similarly..
Graphical method
for graphical method we will have to again use the critical point..
at critical point, it is the lowest value of mod and on either side it increases with a negative slope on one side and positive slope on other side
so it forms a 'V' shape in linear equation and a 'U ' curve for Quadratic Equation..
If the mod has a negative sign in front, -|x+3|, it will have an "inverted V" shape with max value at critical value..
lets see the three examples..
a) only one Mod
eg.. |x+2| + 2x= 3..
critical value at -2 and equation can be written as
|x+2| = 3-2x..
we take y=|x+2| and draw a graph and then take y=3-2x and again draw graph..
the point of intersection is our value..
b) two mods..
|x+2|=|x-3|+1..
here we will take y=|x+2| and y=|x-3|+1
again the point of intersection of two sides will give us the value of x..
c) three mods.. very rare
|x+3|-|4-x|=|8+x| ..
Here we have three critical values, but the graph will still be only two, one for LHS and one for RHS..
It will not be three for three mod as someone has drawn it in one of the discussions on this Q..
again we see the intersection of two graph..
there are no points of intersection , so no solution
THE FINER POINT
1) Opening modulus is time consuming, susceptible to error, and the answer found can still be wrong and has to checked by putting the values in mod again..
should be least priority and should be used by someone has not been able to grasp finer points of other two methods..
2) "Critical method" should be the one used in most circumstances although it requires a good understanding of signs given to the mod when opened within a region.
It has to be the method, when you are looking for values of X..
3) "Graphical method" is useful in finding the number of values of x, as getting accurate values of x may be difficult while estimating from free hand graphs..
but if understood much faster and easier to find sol for Q like How many solutions does the equation |x+3|-|4-x|=|8+x| have?....
Hope it helps atleast a few of you..
Attachments
File comment: |x+3|-|4-x|=|8+x| ..
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File comment: |x+2|=|x-3|+1..
docu2.png [ 40.42 KiB | Viewed 162479 times ]
_________________
periyar22
Mr periyar,
Firstly, these are the methods used to solve modulus. So, if you find them confusing, you will have to learn them or leave the topic or invent a new method to solve it.
Secondly this post is related to quant, and I hope you are not mistaking this to be a verbal post, as you seem to be more interested in spellings. Thankfully, you found only one mistake in the big post above. But it is surprising, you could not avoid a mistake in even a single line you have written in last two years. It is not cant but can't.
Anyway, experts badge does not mean anyone should follow what he writes and in the same way, it is not necessary if an intelligent person but without a expert badge, like you, writes people will not follow. Please use this forum and assist others and surely people will follow you more than they follow any expert.
By the way, I will not edit rare and rear as it is a mark of your keen observation.
_________________
udbhav2412
In addition to Chetan's excellent post on absolute values, you can find the following topics useful:
Theory on Absolute Values: math-absolute-value-modulus-86462.html
The E-GMAT Question Series on ABSOLUTE VALUE: the-e-gmat-question-series-on-absolute-value-198503.html
Properties of Absolute Values on the GMAT: properties-of-absolute-values-on-the-gmat-191317.html
Absolute Value: Tips and hints: absolute-value-tips-and-hints-175002.html
DS Absolute Values Questions to practice: search.php?search_id=tag&tag_id=37
PS Absolute Values Questions to practice: search.php?search_id=tag&tag_id=58
Hard set on Absolute Values: inequality-and-absolute-value-questions-from-my-collection-86939.html
The E-GMAT Question Series on ABSOLUTE VALUE: the-e-gmat-question-series-on-absolute-value-198503.html
Properties of Absolute Values on the GMAT: properties-of-absolute-values-on-the-gmat-191317.html
Absolute Value: Tips and hints: absolute-value-tips-and-hints-175002.html
DS Absolute Values Questions to practice: search.php?search_id=tag&tag_id=37
PS Absolute Values Questions to practice: search.php?search_id=tag&tag_id=58
Hard set on Absolute Values: inequality-and-absolute-value-questions-from-my-collection-86939.html
For other topics please refer to ALL YOU NEED FOR QUANT ! ! !.
Hope it helps.
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gmatexam439
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Thank you Chetan for this. I would like to share an alternate method for solving absolute equations which can be used for as many variables: [I used this while persuing my engg.]
While solving the below equation:
|x+3|-|4-x|=|8+x|
=> |x+3|-|4-x|-|8+x|=0
Therefore the critical points will be x=-8,-3 and 4
Case 1. Now when x<-8 on the number line
|x+3| will open with a -ve sign, |4-x| will open with a +ve sign and |8+x| will open with a -ve sign
Therefore, equation will be
=> -x-3-4+x+8+x=0
=> x=-1; Now if we notice this value of x doesn't match with the case value i.e. x<-8 therefore this is not a correct answer.
Case 2. Now when -8<x<-3 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a +ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3-4+x-8-x=0
=> x=9; Now if we notice this value of x doesn't match with the case value i.e. -8<x<-3 therefore this is not a correct answer.
Case 3. Now when -3<x<4 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a +ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3-4+x-8-x=0
=> x=9; Now if we notice this value of x doesn't match with the case value i.e. -3<x<4 therefore this is not a correct answer.
Case 4. Now when x>4 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a -ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3+4-x-8-x=0
=> x=-1; Now if we notice this value of x doesn't match with the case value i.e. x>4 therefore this is not a correct answer.
Therefore there is no solution to this equation.
While solving the below equation:
|x+3|-|4-x|=|8+x|
=> |x+3|-|4-x|-|8+x|=0
Therefore the critical points will be x=-8,-3 and 4
Case 1. Now when x<-8 on the number line
|x+3| will open with a -ve sign, |4-x| will open with a +ve sign and |8+x| will open with a -ve sign
Therefore, equation will be
=> -x-3-4+x+8+x=0
=> x=-1; Now if we notice this value of x doesn't match with the case value i.e. x<-8 therefore this is not a correct answer.
Case 2. Now when -8<x<-3 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a +ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3-4+x-8-x=0
=> x=9; Now if we notice this value of x doesn't match with the case value i.e. -8<x<-3 therefore this is not a correct answer.
Case 3. Now when -3<x<4 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a +ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3-4+x-8-x=0
=> x=9; Now if we notice this value of x doesn't match with the case value i.e. -3<x<4 therefore this is not a correct answer.
Case 4. Now when x>4 on the number line
|x+3| will open with a +ve sign, |4-x| will open with a -ve sign and |8+x| will open with a +ve sign
Therefore, equation will be
=> x+3+4-x-8-x=0
=> x=-1; Now if we notice this value of x doesn't match with the case value i.e. x>4 therefore this is not a correct answer.
Therefore there is no solution to this equation.
General Discussion
udbhav2412
Joined: 22 Jan 2015
Last visit: 16 Aug 2017
Posts: 12
Location: India
Concentration: Technology, General Management
Posts: 12
nice post and exactly what i was looking for 
udbhav2412
Joined: 22 Jan 2015
Last visit: 16 Aug 2017
Posts: 12
Location: India
Concentration: Technology, General Management
Posts: 12
Bunuel
Thanks a lot Brunel
these gentlemen Chetan2u and Bunuel are fab peeps i just discovered this pot of gold! Thanks Chetan2u given kudos for this!
nipunjain14
Joined: 07 May 2015
Last visit: 12 Aug 2017
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Location: India
Schools: ISB '19 Kellogg '20 Yale '20 HEC Dec"18
GMAT 1: 660 Q48 V31
GPA: 3
Thanks Chetan for compiling this.
I have realized GMAT is all about concepts, tips and tricks. Surely this is good one.
I have realized GMAT is all about concepts, tips and tricks. Surely this is good one.
Posts: 66
Thanks Chetan, this helps a lot, especially since my D-day is fast approaching.
I had 2 questions though.
1. (A very basic one) When it comes to inequalities with mod, say | X + 1 | > 6, then, when we open the mod which of these is the correct expression?
A. X + 1 > 6 or X + 1 > - 6
B. X + 1 > 6 or X + 1 < - 6 (Basically do we flip the sing when we take the negative?)
2. Also, does the 'critical number' and 'open the modulus' apply for quadritic equations and inequalities as well, or de we have to rely only on the graph technique?
I had 2 questions though.
1. (A very basic one) When it comes to inequalities with mod, say | X + 1 | > 6, then, when we open the mod which of these is the correct expression?
A. X + 1 > 6 or X + 1 > - 6
B. X + 1 > 6 or X + 1 < - 6 (Basically do we flip the sing when we take the negative?)
2. Also, does the 'critical number' and 'open the modulus' apply for quadritic equations and inequalities as well, or de we have to rely only on the graph technique?
Can you please explain how the 3 mod equation is graphed on detail?
thanks man
Posted from my mobile device
thanks man
Posted from my mobile device
Posts: 29
Hi Chetan,
Thankfully , i bumped into this post of yours',while reading your reply to my query on SC.
I was looking for such an explanation on Mods.I find this Topic on QA rather confusing.
Will take time out to to understand your explanation
Thanks Again
Thankfully , i bumped into this post of yours',while reading your reply to my query on SC.
I was looking for such an explanation on Mods.I find this Topic on QA rather confusing.
Will take time out to to understand your explanation
Thanks Again
chetan2u
Posts: 29
Hi Bunuel,
Super thanks for the additional links on MODs
Regards,
Abhishek
Super thanks for the additional links on MODs
Regards,
Abhishek
Bunuel
Hi Chetan,
First of all, thanks for sharing this information. In graphical method, I could not get how did you draw the graph for two modes at one side (x+3|-|4-x|) . My question might be silly but I could not get it. Please explain. Thanks once again.
First of all, thanks for sharing this information. In graphical method, I could not get how did you draw the graph for two modes at one side (x+3|-|4-x|) . My question might be silly but I could not get it. Please explain. Thanks once again.
AR15J
Hi,
The LHS is|x+3|-|4-x|...
Take this to be equal to y, so y=|x+3|-|4-x|•••••
Now find y for different values of x..
When x=0, y=|0+3|-|4-0|=-1...
When x=1, y=|1+3|-|4-1|=1...
For all values of x as 7 & above 7, y will be a constant 7... and below -7, y will be -7...
You can draw the graph with these points..
_________________
Beautifully explained. Can't thank you enough. THANKYOU !!
Chetan Very well explained. Hope we have similar summary for all GMAT question types and it would be Awesome reference guide. Great effort. Kudos to you.
Amazing thank you! Kudos to you
@
Awesome post.This is what i was looking for.
One small doubt. Can you please explain how we can simplify |4-x|? I think we can write it like |x-4|, but want to understand the concept that why we can write it like this?
chetan2u
Awesome post.This is what i was looking for.
One small doubt. Can you please explain how we can simplify |4-x|? I think we can write it like |x-4|, but want to understand the concept that why we can write it like this?
nishantt7
That is correct.
If you keep the modulus sign intact =>Then absolutely we can.
In case you decide to remove the modulus sign-->
|4-x| => 4-x for x<4
=> x-4 for x>4
=> 4 for x=4
AkshayKS21
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GRE 1: Q163 V154
Posts: 57
I wonder why I didn't come across this before...
Brilliant post.
I used only first method-always, and that is why struggled with 2 and 3 mod questions.
And amazing work on "How many solutions does the equation with more than 1 mod have". Made my life easier.
A big big big Thank You!
Brilliant post.
I used only first method-always, and that is why struggled with 2 and 3 mod questions.
And amazing work on "How many solutions does the equation with more than 1 mod have". Made my life easier.
A big big big Thank You!
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