Absolutely Outrageous

Algebra Level 2

The set of solutions of the equation x 1 + 1 1 = x + 1 1 + 1 \bigg\lvert \Big\lvert \lvert x-1 \rvert + 1 \Big\rvert - 1 \bigg\rvert = \bigg\lvert \Big\lvert \lvert x+1 \rvert - 1 \Big\rvert + 1 \bigg\rvert is a disjoint union of one or more segments. Find the sum of their lengths.


The answer is 1.

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5 solutions

Samuel Hatin
Oct 13, 2013

The first member of the equation given can be rewritten like this :

(I will write the domain in brackets, I find it easier to type on the computer)

x 1 + 1 1 = x 1 \left| {\left| {\left| {x - 1} \right| + 1} \right| - 1} \right| = \left| {x - 1} \right|

The right member can be written as a composite function :

x + 1 1 + 1 = { x 1 [ x < 2 ] x + 3 [ 2 x < 1 ] x + 1 [ 1 x < 0 ] x 1 [ x 0 ] \left| {\left| {\left| {x + 1} \right| - 1} \right| + 1} \right| = \left\{ \begin{array}{l} - x - 1\left[ {x < - 2} \right]\\ x + 3\left[ { - 2 \le x < - 1} \right]\\ - x + 1 \left[ { - 1 \le x < 0} \right]\\ x - 1\left[ { x \ge 0} \right] \end{array} \right.

The left member can also be written like this :

x 1 = { x + 1 [ x < 1 ] x 1 [ x 1 ] \left| {x - 1} \right| = \left\{ \begin{array}{l} - x + 1\left[ {x < 1} \right]\\ x - 1\left[ {x \ge 1} \right] \end{array} \right.

We see that the third equation of the right member is the same as the first equation of the left member.so we have :

x + 1 [ x < 1 ] = x + 1 [ 1 x < 0 ] - x + 1\left[ {x < 1} \right] = - x + 1\left[ { - 1 \le x < 0} \right]

Then the solution is the line x + 1 - x + 1 on x = [ 1 , 0 ] x = \left[ { - 1,0} \right] and the length of the only solution is 1.

you can simply plot their graphs. Like the LHS is x 1 + 1 1 |||x-1|+1|-1| you can plot it using simple transformations. similarly plot the RHS and their intersection will provide the solution set

Deepansh Mathur - 7 years, 8 months ago

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solving via graph is too easy and comfortable :-)

Piyush Golani - 7 years, 7 months ago

you made this more complicated than it needs to be

Quinn Hail - 7 years, 8 months ago

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How would you present a simpler solution?

Calvin Lin Staff - 7 years, 7 months ago

how to solve it using graphs?

Cody Martin - 7 years, 7 months ago

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You'll have to plot the graphs of those functions using transformations. Let's take the LHS .

We have x 1 + 1 1 |||x-1|+1|-1| .

Now its obvious that this function is a composite function.

Now lets look at the simplest function in this composite function.

x 1 \rightarrow |x-1|

Let it be f(x)

Now Its easy to plot f(x)

<http://www.wolframalpha.com/input/?i=plot+%7Cx-1%7C>

now the function becomes f ( x ) + 1 1 ||f(x)+1|-1| as x 1 |x-1| was replaced by f(x)

And now we will be doing operations on f(x) instead of x

So now the next simplest function we can see is f ( x ) + 1 |f(x)+1|

Now you already have the plot of f(x) All you gotta do to make the graph of f ( x ) + 1 |f(x)+1| is Push f(x) 1 unit up on the Y axis this will give f(x)+1

here it is

<http://www.wolframalpha.com/input/?i=plot+%7Cx-1%7C+%2B1>

And this leads to our next step, Taking absolute of this function.

i.e, we have to make f ( x ) + 1 |f(x)+1| now.

As you can know, the absolute function converts all the -ve values to +ve values.

So the best method to make the graph of g ( x ) |g(x)| is to flip the negative portion of the graph of g(x) into the positive part taking X axis as the hinge.

here g(x) is a random function

you can try it on your own, let g(x)=x , now make g ( x ) = x |g(x)| = |x|

you'll see that the -ve portion of y = x y=x was flipped on the +ve y axis side

So the function we had was f ( x ) + 1 |f(x)+1| , you already have f(x)+1 and using the method i told you above, make f ( x ) + 1 |f(x)+1| .

Its obvious that the graph of f ( x ) + 1 |f(x)+1| will be the same as of f ( x ) + 1 f(x)+1

as there is no -ve part in f ( x ) + 1 f(x)+1

here it is

<http://www.wolframalpha.com/input/?i=plot+%7C%7Cx-1%7C+%2B1%7C>

OK now we have the graph of x 1 + 1 ||x-1|+1|

let x 1 + 1 = h ( x ) ||x-1|+1| = h(x) ,

so the objective now is to make the graph of h ( x ) 1 |h(x)-1|

so again, its simple, push down h(x) 1 unit on y axis

<http://www.wolframalpha.com/input/?i=plot+%7C%7Cx-1%7C+%2B1%7C+-1+>

and take the absolute of this, i.e. flip the -ve part of the plot to +ve part.

obviously if there's no -ve part then the graph will not change

you'll get this h ( x ) 1 |h(x)-1| or x 1 + 1 1 |||x-1|+1|-1|

Similarly plot the graph of the function on the RHS

and where ever they will intersect will give us the soution

if you need help with this or how to make the graph of the function on rhs, let me know, i'll comment the solution to that

Deepansh Mathur - 7 years, 7 months ago

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This was a very helpful, clear explanation and a much simpler way of going about solving this - thanks!

Eva Donlon - 7 years, 7 months ago

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@Eva Donlon No worries! I am glad my solution helped you! :)

Deepansh Mathur - 7 years, 7 months ago

we can plot the two graphs and see the intersection of domain

Goutam Narayan - 7 years, 7 months ago

gosh that was a very complex solution.

HeurEux K-ann - 6 years, 10 months ago

the answer is -1

Kaushik Srinivasan - 6 years, 10 months ago

I did it graphically....sol region was [-1,0]...which has a length of 1..

Bhumesh Tanwar - 6 years, 10 months ago

From getting that solution is on the line, how did you get [- 1,0]?

Rajat Gupta - 6 years, 5 months ago

Last part of the right 'member' should be x+1... otherwise the equation would be true for all x≥0 !

Davy Ker - 5 years, 5 months ago

Hi Samuel Hatin, how do u know u can simplify LHS to |x-1| directly? Any workings needed? And how do u determine the critical values for the RHS so quickly? Tq

JOHNNY CHIN - 5 years, 1 month ago

try to solve it via graph it is quite easier than wat u did

Saurabh Dabral - 7 years, 7 months ago

i got this the answer is -1=0=-1

Kaushik Srinivasan - 6 years, 10 months ago
Andreas Wendler
Nov 21, 2015

First we see by testing that x=0 and x=-1 are solutions.

Furthermore we confirm that all members of [-1 , 0] solve the equation, because we obtain 1-x = 1-x which is correct for all x.

The interval x>0 delivers 1-x = 1+x for x<=1 with the only "solution" x=0 which is no member of the interval considered and x-1 = 1+x for x>1 as a contradiction. Therefore no solution is to be found for x>0.

For interval x<-1 the equation has to be written 1-x = -1-x for x<=-2 which is a contradiction and 1-x = 3+x for x>-2 with irrelevant "solution" x=-1 since this value is outside the interval. Again we can not detect any solution for x<-1.

So we proved that the only segment [-1 , 0] with length 1 is responsible for the infinite set of solutions of this cute problem.

Ashwin Anand
May 2, 2014

lhs is simply l x- 1 l for rhs, use graphical trans to get graph, it should appear like a W 1 unit above x-axis with centre at x=-1. u will see that they coincide only in the interval (-1,0)

Snehdeep Arora
Oct 14, 2013

solving we get x = 0 x=0 and x = 1 x=-1 which gives us x ϵ ( 1 , 0 ) x \epsilon (-1,0)

length = 0 ( 1 ) = 1 0-(-1) = \boxed1

How did you solve it? Also, if the solution is only 2 points as opposed to a line segment, then the sum of the lengths is 0.

Calvin Lin Staff - 7 years, 7 months ago

oops, it is x = [ 1 , 0 ] x = [-1,0]

Snehdeep Arora - 7 years, 8 months ago
Santanu Banerjee
Oct 14, 2013

Answer is the length of the segment :; [ -1 , 0 ]

Why?

Calvin Lin Staff - 7 years, 7 months ago

Why the right hand side interval have written x < - 2.. Really confused!

pranay pal - 4 years ago

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