|2x+3||x+6|

Question

|2x+3|>|x+6|

please solve

Fig · Answer

|2x+3|>|x+6|

In order to find the solution by simplifying the inequation (no absolute), we have to work on the following domains:
o x =< -6
o -6 < x < -3/2
o x >= -3/2

o If x =< -6, the inequation is simplified to:

-(2*x+3) > -(x+6)
<=> x < 3

Thus, all values of x such that x =< -6 are solutions.

o If -6 < x < -3/2, the inequation is simplified to:

-(2*x+3) > (x+6)
<=> 3*x < -9
<=> x < -3

Thus, all values of x such that -6 < x < -3 are solutions.

o If x >= -3/2, the inequation is simplified to:

(2*x+3) > (x+6)
<=> x > 3

Thus, all values of x such that x > 3 are solutions.

Finally, we add all solutions :
x < -3 or x > 3

800_gal · Answer

Hi Fig Can you please explain how do you find the domains. Thanks

hobbit · Answer

actually, for this question there is a simpler way to solve....

since both sides of the equation are positive, we can safely square them 
(in general, for positive a,b a>b if and only if a^2>b^2)

getting in this case:

4x^2+12x+9 > x^2+12x+36 which simplifies to:

x^2 > 9

which yields easily the result Fig got.

Fig · Answer

Basically, we look for the "crucial" points that flip the sign in every abs.

Here, we have 2 abs and so 2 point to find by equalling them to 0: > 2*x+3 = 0 <=> x = -3/2 > x+6 = 0 <=> x = -6 After that, the domain definied from those points. o x =< -6 o -6 < x < -3/2 o x >= -3/2

subhen · Answer

Hi,

I found Hobbit's method of squaring both sides and finding the solution faster and easier. Frankly Fig's method looks a bit daunting. I am still not clear about the concept of domains for absolute values as suggested by Fig.

The way I approached the problem was the traditional way of using all 4 possibilities for setting up absolute values in an inequality.

eg. what I did was I used the following 4 equations.
(1) 2x + 3 > x + 6 yields x > 3
(2) -2x - 3 > x + 6 yields x < -3
(3) 2x + 3 > -x - 6 yields x > -3
(4) -2x - 3 > -x - 6 yields x < 3

Using the more restrictive inequality we arrive at x > 3 or x < -3.

Fig · Answer

The way that hobbit uses is good for this question

... Not for all

... And, I do agree that there are different ways to solve the same problem

Personnally, I prefer the graphical approach

.... Drawing fastly an XY plan with the courbs and the important points determined by few calculation if required

To come back to your inequations : the (3) and (4) does not represent the original ineqation. There is no reason for the right side to be adapted to -x.

hobbit · Answer

i never tried to claim my method is generic....
in fact it is good only if you know that both sides are positive. this is a special case.

there is also another way that works:
instead of solving something > something else
solve first: something = something else
and then sample (pick numbers) in each region.

for example:
to solve |2x+3|>|x+6|
we start by solving
|2x+3|=|x+6|
there are two cases:
2x+3=x+6 => x=3
or
2x+3= -(x+6) => x=-3

so we have three regions: x<-3 , -3<x<3 and x>3
pick number for each region:
try x=-4 => we see that |2x+3|=|-5|=5 > |x+6| = |-2| = 2
try x=0 => we see that |2x+3| = 3 < |x+6|=6
try x=4 => we see that |2x+3| = 11 > |x+6|=10

so there are two regions that works and this is the solution:
x<-3 or x>3

the reason why checking one point of the region is enough to infer that the whole region share the property is beyond the scope of the discussion here (i can tell you if you really want)...

but this is a good method for solving inequalities in almost all cases.

Fig · Answer

I must disagree here... One more time, it works on specific question. But, what about the others?

Could u define the scope of this method ?... In these cases, people will use this short cut

hobbit · Answer

actually this method works for virtually any inequality !!!!!! (the real condition for it to work is that both sides will be continuous functions...for those who know the concept.... for practical uses - it is every inequality that is defined for all numbers) i'll explain more.... the methods work like this: to solve an inequality: expression1>expression2, where both expressions are defined for all x. a) solve the equation: expression1=expression2 b) put the solutions on the number line. this divide it into segments. c) choose one number for each segment created. for each such value - check the original inequality. if it holds - then the whole segment solves the inequality. the solution is the union of all segments whose representative "passed" the test in stage c. the reason this method works is this - suppose i didn't work. this means that there exists two numbers a and b in the same segment for one exp1>exp2 and for the other exp1exp2 for b for every a,b in the same segment. so it is sufficient to check one number for each segment to check whether the original inequality holds for the segment. this method can be extended to work for expressions that have some points where they are undefined..... and to calm all people down.... most if not all expressions that you'll get into in life are continuous. so the method is quite robust.

hobbit · Answer

ok... lets try solving this in "my method"... the inequation is |x+2|>X^2 so first we solve: |x+2|=x^2 for x>-2 we get: x+2=x^2 => x=2 or x=-1 for x<-2: -x-2=x^2 which has no solution. so our solutions to the equation divide the number line into 3 segments: x<-1 -12 checking the first (using x=-2) we see that the inequation doesn't hold checking the middle segment (using x=0) we see that the inequation holds checking the last segment (using x=3) the inequation doesn't hold. so the solution is the middle segment. -1

Fig · Answer

Sorry hobbit

... I had deleted my post

.... An example should be this one and I follow your way : |x^2 - 4| > x + 2. So, x^2 - x - 6 = 0.... roots = -2 and 3. That implies, we study on x < -2, -2 < x < 3 and x > 3. So, in the case of -2 < x < 3, picking 1 number is sufficient. We could take 2. |2^2 - 4| = 0 < 2 + 2 = 4. So, the inequality is never verified on -2 < x < 3 ?.... The answer is no. Why? X = -1 >>> |-3| = 3 > -1+2 = 1.

hobbit · Answer

you just missed one solution for the equation....

|x^2-4|=x+2 has 3 solutions: x=3 x=1 and x=-2

so there are 4 segments to check - not three.

note that you checked x=2 and tried to give a counter example with x=-1. given that x=1 is also a solution for the equation, 2 and -1 are not in the same segment - so no contradiction.

"my method" still works.... (and i'm quite sure it will be very difficult to find a counter example).

Fig · Answer

Ahhh... I c. U preserve the absolute to solve... A part, I must say, I didnt see.

Yes.... u are checking the sign (table of sign of the function https://www.gmatclub.com/phpbb/viewtopic.php?t=36468 ) by empirical values. It works yes.... Agree

So, your methodology is useful for cases that do not contain too much absolute values, that should be the GMAT cases.

In longer cases, |x+8| - |x-7| > |x+1| + |x-1| equalling makes it hard to solve.

Andr359 · Answer

If you feel comfortable plotting lines on the xy plane, you could solve this inequality just by inspecting the intersections of both absolute value functions. The result is 

x < -3 AND x > 3.

riyakajaria1984 · Answer

I didn't any solution. There can be four situations:

First, when |2x+3|is positive and |x+6| is positive
=> 2x+3 > x+6
=> x > 3

Second, when |2x+3|is positive and |x+6| is negative
=> 2x+3 > -x-6
=> 3x > -9
=> x > -3

Third, when |2x+3|is negative and |x+6| is positive
=> -2x-3 > x+6
=> -3x > 9
=> x < -3

Fourth, when |2x+3|is negative and |x+6| is negative
=> -2x-3 > -x-6
=> -3x > -9
=> x < 3

Now, combining all four, we have no possible solution. Can anybody pls explain how to solve this by opening modules? :hammer

metallicafan · Answer

My answer: x > 3 and x < -3 Previously to that answer, I got the possible solutions: x > 3 , x < 3 , x < -3 , and x > -3 . So, I replaced these values in the original inequlity to see whether all these solutions confirmed that relationship. Only, the first and third solution did so. Is my approach correct? Thank you.

devinawilliam83 · Answer

Am unable to understand how the solutions have been combined to get x<-3 or x>3.. The solution does not take into account the limits set by -6 in x<-6 or -6<x<-3.. Please Help

abhi47 · Answer

You can solve this question using Mgmat equalities approach which is very easy.

Bunuel · Answer

Approach #1: Fig's approach. |2x+3|>|x+6| We have two check points: -6 and -\frac{3}{2} (check point the value fox for which an expression in modulus equals to zero). Thus 3 ranges to check: 1. x\leq{-6} --> in this range 2x+3<0 and x+6<0 thus absolute values expand as: -(2x+3)>-(x+6) --> x<3 . Now, since we consider the range x\leq{-6} then the solution for this range will be x<-6 : --------(-6) ------------(-3/2)-------- 2. -6 in this range 2x+3<0 and x+6>0 thus absolute values expand as: -(2x+3)>(x+6) --> x<-3 . Since we consider the range -6 in this range 2x+3>0 and x+6>0 thus absolute values expand as: (2x+3)>(x+6) --> x>3 . Since we consider the range -\frac{3}{2}\leq{x} then the solution for this range will be x>3 : --------(-6)----(-3)----(-3/2)--------3 -------- Combined solution is: --------(-6)---- (-3)----(-3/2)--------3 -------- x<-3 or x>3 . Approach #2: Easier one. |2x+3|>|x+6| Since both sides of the inequality are nonnegative then we can safely square it: (2x+3)^2>(x+6)^2 --> 4x^2+12x+9>x^2+12x+36 --> 3x^2>27 --> x^2>9 --> x<-3 or x>3 . Hope it helps.

|2x+3|>|x+6|

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards