Absolute Value Inequality (another)












2












$begingroup$



Solve the following inequality



$$|a-2| > |a+4|$$




Here I separated it into cases as shown




$a<-4$




$$-(a-2) > -(a+4) implies 2-a>-a-4 implies 0>-6$$



Always true, so we get $mathbb{R} cap (-infty , -4) = (-infty ,-4)$




$-4<a<2$




$$-(a-2)>a+4 implies 2-a>a+4 implies a<-1$$



Taking interception $(-4,2)$ $cap $ $(-infty,-1)$




$a>2$




$$a-2 > a+4 implies -2>4 $$



Always false, so no solution from there. Finally, I checked end points and noticed that they do not work in this inequality. However, I do not know how to proceed further. Could you assist me?



Regards










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$endgroup$












  • $begingroup$
    With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
    $endgroup$
    – DonAntonio
    Jan 13 at 13:18






  • 1




    $begingroup$
    @DonAntonio Not really, thanks for pointing out.
    $endgroup$
    – Enzo
    Jan 13 at 13:19
















2












$begingroup$



Solve the following inequality



$$|a-2| > |a+4|$$




Here I separated it into cases as shown




$a<-4$




$$-(a-2) > -(a+4) implies 2-a>-a-4 implies 0>-6$$



Always true, so we get $mathbb{R} cap (-infty , -4) = (-infty ,-4)$




$-4<a<2$




$$-(a-2)>a+4 implies 2-a>a+4 implies a<-1$$



Taking interception $(-4,2)$ $cap $ $(-infty,-1)$




$a>2$




$$a-2 > a+4 implies -2>4 $$



Always false, so no solution from there. Finally, I checked end points and noticed that they do not work in this inequality. However, I do not know how to proceed further. Could you assist me?



Regards










share|cite|improve this question











$endgroup$












  • $begingroup$
    With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
    $endgroup$
    – DonAntonio
    Jan 13 at 13:18






  • 1




    $begingroup$
    @DonAntonio Not really, thanks for pointing out.
    $endgroup$
    – Enzo
    Jan 13 at 13:19














2












2








2





$begingroup$



Solve the following inequality



$$|a-2| > |a+4|$$




Here I separated it into cases as shown




$a<-4$




$$-(a-2) > -(a+4) implies 2-a>-a-4 implies 0>-6$$



Always true, so we get $mathbb{R} cap (-infty , -4) = (-infty ,-4)$




$-4<a<2$




$$-(a-2)>a+4 implies 2-a>a+4 implies a<-1$$



Taking interception $(-4,2)$ $cap $ $(-infty,-1)$




$a>2$




$$a-2 > a+4 implies -2>4 $$



Always false, so no solution from there. Finally, I checked end points and noticed that they do not work in this inequality. However, I do not know how to proceed further. Could you assist me?



Regards










share|cite|improve this question











$endgroup$





Solve the following inequality



$$|a-2| > |a+4|$$




Here I separated it into cases as shown




$a<-4$




$$-(a-2) > -(a+4) implies 2-a>-a-4 implies 0>-6$$



Always true, so we get $mathbb{R} cap (-infty , -4) = (-infty ,-4)$




$-4<a<2$




$$-(a-2)>a+4 implies 2-a>a+4 implies a<-1$$



Taking interception $(-4,2)$ $cap $ $(-infty,-1)$




$a>2$




$$a-2 > a+4 implies -2>4 $$



Always false, so no solution from there. Finally, I checked end points and noticed that they do not work in this inequality. However, I do not know how to proceed further. Could you assist me?



Regards







inequality






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share|cite|improve this question













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edited Jan 13 at 13:34









Ethan Bolker

43.1k551114




43.1k551114










asked Jan 13 at 13:16









EnzoEnzo

19917




19917












  • $begingroup$
    With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
    $endgroup$
    – DonAntonio
    Jan 13 at 13:18






  • 1




    $begingroup$
    @DonAntonio Not really, thanks for pointing out.
    $endgroup$
    – Enzo
    Jan 13 at 13:19


















  • $begingroup$
    With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
    $endgroup$
    – DonAntonio
    Jan 13 at 13:18






  • 1




    $begingroup$
    @DonAntonio Not really, thanks for pointing out.
    $endgroup$
    – Enzo
    Jan 13 at 13:19
















$begingroup$
With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
$endgroup$
– DonAntonio
Jan 13 at 13:18




$begingroup$
With $;a=4;$ your inequality is $$2=|4-2|>|4+4|=8...text{you still think this is true?}$$
$endgroup$
– DonAntonio
Jan 13 at 13:18




1




1




$begingroup$
@DonAntonio Not really, thanks for pointing out.
$endgroup$
– Enzo
Jan 13 at 13:19




$begingroup$
@DonAntonio Not really, thanks for pointing out.
$endgroup$
– Enzo
Jan 13 at 13:19










4 Answers
4






active

oldest

votes


















2












$begingroup$

As commented, $;a=4;$ doesn't really fits in the inequality. You can now put your solution set as



$$(-infty,-4)cupleft((-4,-2)cap(-infty,-1)right)=(-infty,-4)cup(-4,-1)=(-infty,-1)setminus{-4}$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    What does this mathematical symbol mean ?
    $endgroup$
    – Enzo
    Jan 13 at 13:23












  • $begingroup$
    Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
    $endgroup$
    – DonAntonio
    Jan 13 at 13:23










  • $begingroup$
    The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
    $endgroup$
    – Enzo
    Jan 13 at 13:24












  • $begingroup$
    That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
    $endgroup$
    – DonAntonio
    Jan 13 at 13:34





















2












$begingroup$

If you interpret the absolute values in terms of distance, it is immediate:



$|a-2|>|a+4|$ means $a$ is nearer to $-4$ than to $2$, so it is less than the arithmetic mean of $2$ and $-4$:
$$a<frac{2-4}2=-1$$



Other method:
begin{align}
|a-2|>|a+4|&iff(a-2)^2>(a+4)^2 \
&iff a^2-4a+4>a^2+8a+16\
&iff 12a+12<0iff a<-1.
end{align}






share|cite|improve this answer











$endgroup$





















    1












    $begingroup$

    You have to distinguish the following cases:
    $$xgeq 2$$ then we get $$x-2>x+4$$
    $$-4le x<2$$ then we get $$x-2>-x+4$$
    $$x<-4$$ then we have $$-x+2>-x-4$$






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      $|(a+1)-3|>|(a+1)+3|;$



      $x:=a+1$;



      $|x-(+3)| >|x-(-3)|;$



      Real number line :



      Distance from a point $x$ to $(+3)$ is bigger than from $x$
      to $(-3)$, i.e. $x<0.$



      (Note : At $x=0$ both distances are equal, for $x <0$, $x$ is to the left of $0$ , closer to $(-3)$ ).



      $x= a+1<0$, $a<-1$.






      share|cite|improve this answer









      $endgroup$













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        4 Answers
        4






        active

        oldest

        votes








        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        2












        $begingroup$

        As commented, $;a=4;$ doesn't really fits in the inequality. You can now put your solution set as



        $$(-infty,-4)cupleft((-4,-2)cap(-infty,-1)right)=(-infty,-4)cup(-4,-1)=(-infty,-1)setminus{-4}$$






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          What does this mathematical symbol mean ?
          $endgroup$
          – Enzo
          Jan 13 at 13:23












        • $begingroup$
          Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
          $endgroup$
          – DonAntonio
          Jan 13 at 13:23










        • $begingroup$
          The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
          $endgroup$
          – Enzo
          Jan 13 at 13:24












        • $begingroup$
          That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
          $endgroup$
          – DonAntonio
          Jan 13 at 13:34


















        2












        $begingroup$

        As commented, $;a=4;$ doesn't really fits in the inequality. You can now put your solution set as



        $$(-infty,-4)cupleft((-4,-2)cap(-infty,-1)right)=(-infty,-4)cup(-4,-1)=(-infty,-1)setminus{-4}$$






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          What does this mathematical symbol mean ?
          $endgroup$
          – Enzo
          Jan 13 at 13:23












        • $begingroup$
          Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
          $endgroup$
          – DonAntonio
          Jan 13 at 13:23










        • $begingroup$
          The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
          $endgroup$
          – Enzo
          Jan 13 at 13:24












        • $begingroup$
          That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
          $endgroup$
          – DonAntonio
          Jan 13 at 13:34
















        2












        2








        2





        $begingroup$

        As commented, $;a=4;$ doesn't really fits in the inequality. You can now put your solution set as



        $$(-infty,-4)cupleft((-4,-2)cap(-infty,-1)right)=(-infty,-4)cup(-4,-1)=(-infty,-1)setminus{-4}$$






        share|cite|improve this answer









        $endgroup$



        As commented, $;a=4;$ doesn't really fits in the inequality. You can now put your solution set as



        $$(-infty,-4)cupleft((-4,-2)cap(-infty,-1)right)=(-infty,-4)cup(-4,-1)=(-infty,-1)setminus{-4}$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 13 at 13:22









        DonAntonioDonAntonio

        178k1494230




        178k1494230












        • $begingroup$
          What does this mathematical symbol mean ?
          $endgroup$
          – Enzo
          Jan 13 at 13:23












        • $begingroup$
          Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
          $endgroup$
          – DonAntonio
          Jan 13 at 13:23










        • $begingroup$
          The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
          $endgroup$
          – Enzo
          Jan 13 at 13:24












        • $begingroup$
          That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
          $endgroup$
          – DonAntonio
          Jan 13 at 13:34




















        • $begingroup$
          What does this mathematical symbol mean ?
          $endgroup$
          – Enzo
          Jan 13 at 13:23












        • $begingroup$
          Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
          $endgroup$
          – DonAntonio
          Jan 13 at 13:23










        • $begingroup$
          The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
          $endgroup$
          – Enzo
          Jan 13 at 13:24












        • $begingroup$
          That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
          $endgroup$
          – DonAntonio
          Jan 13 at 13:34


















        $begingroup$
        What does this mathematical symbol mean ?
        $endgroup$
        – Enzo
        Jan 13 at 13:23






        $begingroup$
        What does this mathematical symbol mean ?
        $endgroup$
        – Enzo
        Jan 13 at 13:23














        $begingroup$
        Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
        $endgroup$
        – DonAntonio
        Jan 13 at 13:23




        $begingroup$
        Set difference. Sometimes it is also used $;A-B;$ ... A simple minus sign.
        $endgroup$
        – DonAntonio
        Jan 13 at 13:23












        $begingroup$
        The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
        $endgroup$
        – Enzo
        Jan 13 at 13:24






        $begingroup$
        The right answer seems to be $(-infty, -1)$ according to my textbook. Do we get it from your final interval?:
        $endgroup$
        – Enzo
        Jan 13 at 13:24














        $begingroup$
        That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
        $endgroup$
        – DonAntonio
        Jan 13 at 13:34






        $begingroup$
        That is correct, @Enzo . Your solution didn't even take into account $;a=-4;$ (perhaps you should have done this in the first case). That you have an inequality < or > doesn't mean you must take your cases in the same way
        $endgroup$
        – DonAntonio
        Jan 13 at 13:34













        2












        $begingroup$

        If you interpret the absolute values in terms of distance, it is immediate:



        $|a-2|>|a+4|$ means $a$ is nearer to $-4$ than to $2$, so it is less than the arithmetic mean of $2$ and $-4$:
        $$a<frac{2-4}2=-1$$



        Other method:
        begin{align}
        |a-2|>|a+4|&iff(a-2)^2>(a+4)^2 \
        &iff a^2-4a+4>a^2+8a+16\
        &iff 12a+12<0iff a<-1.
        end{align}






        share|cite|improve this answer











        $endgroup$


















          2












          $begingroup$

          If you interpret the absolute values in terms of distance, it is immediate:



          $|a-2|>|a+4|$ means $a$ is nearer to $-4$ than to $2$, so it is less than the arithmetic mean of $2$ and $-4$:
          $$a<frac{2-4}2=-1$$



          Other method:
          begin{align}
          |a-2|>|a+4|&iff(a-2)^2>(a+4)^2 \
          &iff a^2-4a+4>a^2+8a+16\
          &iff 12a+12<0iff a<-1.
          end{align}






          share|cite|improve this answer











          $endgroup$
















            2












            2








            2





            $begingroup$

            If you interpret the absolute values in terms of distance, it is immediate:



            $|a-2|>|a+4|$ means $a$ is nearer to $-4$ than to $2$, so it is less than the arithmetic mean of $2$ and $-4$:
            $$a<frac{2-4}2=-1$$



            Other method:
            begin{align}
            |a-2|>|a+4|&iff(a-2)^2>(a+4)^2 \
            &iff a^2-4a+4>a^2+8a+16\
            &iff 12a+12<0iff a<-1.
            end{align}






            share|cite|improve this answer











            $endgroup$



            If you interpret the absolute values in terms of distance, it is immediate:



            $|a-2|>|a+4|$ means $a$ is nearer to $-4$ than to $2$, so it is less than the arithmetic mean of $2$ and $-4$:
            $$a<frac{2-4}2=-1$$



            Other method:
            begin{align}
            |a-2|>|a+4|&iff(a-2)^2>(a+4)^2 \
            &iff a^2-4a+4>a^2+8a+16\
            &iff 12a+12<0iff a<-1.
            end{align}







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Jan 13 at 13:51

























            answered Jan 13 at 13:42









            BernardBernard

            121k740116




            121k740116























                1












                $begingroup$

                You have to distinguish the following cases:
                $$xgeq 2$$ then we get $$x-2>x+4$$
                $$-4le x<2$$ then we get $$x-2>-x+4$$
                $$x<-4$$ then we have $$-x+2>-x-4$$






                share|cite|improve this answer









                $endgroup$


















                  1












                  $begingroup$

                  You have to distinguish the following cases:
                  $$xgeq 2$$ then we get $$x-2>x+4$$
                  $$-4le x<2$$ then we get $$x-2>-x+4$$
                  $$x<-4$$ then we have $$-x+2>-x-4$$






                  share|cite|improve this answer









                  $endgroup$
















                    1












                    1








                    1





                    $begingroup$

                    You have to distinguish the following cases:
                    $$xgeq 2$$ then we get $$x-2>x+4$$
                    $$-4le x<2$$ then we get $$x-2>-x+4$$
                    $$x<-4$$ then we have $$-x+2>-x-4$$






                    share|cite|improve this answer









                    $endgroup$



                    You have to distinguish the following cases:
                    $$xgeq 2$$ then we get $$x-2>x+4$$
                    $$-4le x<2$$ then we get $$x-2>-x+4$$
                    $$x<-4$$ then we have $$-x+2>-x-4$$







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Jan 13 at 13:20









                    Dr. Sonnhard GraubnerDr. Sonnhard Graubner

                    75.3k42865




                    75.3k42865























                        1












                        $begingroup$

                        $|(a+1)-3|>|(a+1)+3|;$



                        $x:=a+1$;



                        $|x-(+3)| >|x-(-3)|;$



                        Real number line :



                        Distance from a point $x$ to $(+3)$ is bigger than from $x$
                        to $(-3)$, i.e. $x<0.$



                        (Note : At $x=0$ both distances are equal, for $x <0$, $x$ is to the left of $0$ , closer to $(-3)$ ).



                        $x= a+1<0$, $a<-1$.






                        share|cite|improve this answer









                        $endgroup$


















                          1












                          $begingroup$

                          $|(a+1)-3|>|(a+1)+3|;$



                          $x:=a+1$;



                          $|x-(+3)| >|x-(-3)|;$



                          Real number line :



                          Distance from a point $x$ to $(+3)$ is bigger than from $x$
                          to $(-3)$, i.e. $x<0.$



                          (Note : At $x=0$ both distances are equal, for $x <0$, $x$ is to the left of $0$ , closer to $(-3)$ ).



                          $x= a+1<0$, $a<-1$.






                          share|cite|improve this answer









                          $endgroup$
















                            1












                            1








                            1





                            $begingroup$

                            $|(a+1)-3|>|(a+1)+3|;$



                            $x:=a+1$;



                            $|x-(+3)| >|x-(-3)|;$



                            Real number line :



                            Distance from a point $x$ to $(+3)$ is bigger than from $x$
                            to $(-3)$, i.e. $x<0.$



                            (Note : At $x=0$ both distances are equal, for $x <0$, $x$ is to the left of $0$ , closer to $(-3)$ ).



                            $x= a+1<0$, $a<-1$.






                            share|cite|improve this answer









                            $endgroup$



                            $|(a+1)-3|>|(a+1)+3|;$



                            $x:=a+1$;



                            $|x-(+3)| >|x-(-3)|;$



                            Real number line :



                            Distance from a point $x$ to $(+3)$ is bigger than from $x$
                            to $(-3)$, i.e. $x<0.$



                            (Note : At $x=0$ both distances are equal, for $x <0$, $x$ is to the left of $0$ , closer to $(-3)$ ).



                            $x= a+1<0$, $a<-1$.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 13 at 16:14









                            Peter SzilasPeter Szilas

                            11.3k2822




                            11.3k2822






























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