If $xin[a,b],$ then $|x|leq max{|a|,|b|}$












2












$begingroup$


The following seems to be quite known:




If $xin[a,b],$ then $|x|leq max{|a|,|b|}$




I do use it while treating problems on series of functions but how does it come about? Any hint please?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
    $endgroup$
    – Omojola Micheal
    Dec 29 '18 at 15:50






  • 5




    $begingroup$
    $|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:50
















2












$begingroup$


The following seems to be quite known:




If $xin[a,b],$ then $|x|leq max{|a|,|b|}$




I do use it while treating problems on series of functions but how does it come about? Any hint please?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
    $endgroup$
    – Omojola Micheal
    Dec 29 '18 at 15:50






  • 5




    $begingroup$
    $|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:50














2












2








2


1



$begingroup$


The following seems to be quite known:




If $xin[a,b],$ then $|x|leq max{|a|,|b|}$




I do use it while treating problems on series of functions but how does it come about? Any hint please?










share|cite|improve this question









$endgroup$




The following seems to be quite known:




If $xin[a,b],$ then $|x|leq max{|a|,|b|}$




I do use it while treating problems on series of functions but how does it come about? Any hint please?







real-analysis sequences-and-series absolute-value






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 29 '18 at 15:37









Omojola MichealOmojola Micheal

1,753324




1,753324












  • $begingroup$
    Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
    $endgroup$
    – Omojola Micheal
    Dec 29 '18 at 15:50






  • 5




    $begingroup$
    $|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:50


















  • $begingroup$
    Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
    $endgroup$
    – Omojola Micheal
    Dec 29 '18 at 15:50






  • 5




    $begingroup$
    $|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:50
















$begingroup$
Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
$endgroup$
– Omojola Micheal
Dec 29 '18 at 15:50




$begingroup$
Sorry, can you tell me why you're voting for a close? Perhaps, I'll change somethings.
$endgroup$
– Omojola Micheal
Dec 29 '18 at 15:50




5




5




$begingroup$
$|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
$endgroup$
– Jakobian
Dec 29 '18 at 15:50




$begingroup$
$|x|$ is the distance of $x$ from $0$, it seems obvious that the distance of $x$ from $0$ would be less than the maximum of distance of $a$ and $b$ from $0$.
$endgroup$
– Jakobian
Dec 29 '18 at 15:50










2 Answers
2






active

oldest

votes


















4












$begingroup$

If $xge0$ then $bge0$, hence $$|x|=xle b=|b|lemax(|a|,|b|).$$Similarly if $x<0$ then $a<0$, hence $$|x|=-xle-a=|a|lemax(|a|,|b|).$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:50












  • $begingroup$
    @Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
    $endgroup$
    – David C. Ullrich
    Dec 29 '18 at 15:52












  • $begingroup$
    Very straightforward proof +1
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:56










  • $begingroup$
    @DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:56






  • 2




    $begingroup$
    @Rebellos $x$ is fixed
    $endgroup$
    – Jakobian
    Dec 29 '18 at 16:14



















1












$begingroup$

Since $x in [a,b]$, it is $a<b$ (or even $a leq b$ if you want to be more strict for a trivial case).



If $text{sgn}(a) = text{sgn}(b)=1$, then $max{|a|,|b|} = |b| implies |x| leq max{|a|,|b|}$.



If $text{sgn}(a) = text{sgn}(b)=-1$, then $max{|a|,|b|} = |a| implies |x| leq max{|a|,|b|}$.



In the case of $text{sgn}(a) neq text{sgn}(b)$, that means that $a$ would be negative. But, in that case, it can be $|a| > |b|$. Then, for $|x|$, it would be : $|x| leq |a| implies |x| leq max{|a|,|b|}$.



So, for all cases, we have that : $|x| leq max{|a|,|b|}$.



Edit (Alternativelly) : The comment of Jakobian is really nice to consider for a really straightforward proof. What the absolute value $|x|$ means is the distance of $x$ from $0$. But since $x in [a,b]$, then this distance cannot be larger than the distance of $a$ from zero $(|a|)$ or the distance of $b$ from zero $(|b|)$.






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
    $endgroup$
    – Noble Mushtak
    Dec 29 '18 at 15:45










  • $begingroup$
    @NobleMushtak Sure, edited. I missed a line.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:46













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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









4












$begingroup$

If $xge0$ then $bge0$, hence $$|x|=xle b=|b|lemax(|a|,|b|).$$Similarly if $x<0$ then $a<0$, hence $$|x|=-xle-a=|a|lemax(|a|,|b|).$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:50












  • $begingroup$
    @Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
    $endgroup$
    – David C. Ullrich
    Dec 29 '18 at 15:52












  • $begingroup$
    Very straightforward proof +1
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:56










  • $begingroup$
    @DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:56






  • 2




    $begingroup$
    @Rebellos $x$ is fixed
    $endgroup$
    – Jakobian
    Dec 29 '18 at 16:14
















4












$begingroup$

If $xge0$ then $bge0$, hence $$|x|=xle b=|b|lemax(|a|,|b|).$$Similarly if $x<0$ then $a<0$, hence $$|x|=-xle-a=|a|lemax(|a|,|b|).$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:50












  • $begingroup$
    @Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
    $endgroup$
    – David C. Ullrich
    Dec 29 '18 at 15:52












  • $begingroup$
    Very straightforward proof +1
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:56










  • $begingroup$
    @DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:56






  • 2




    $begingroup$
    @Rebellos $x$ is fixed
    $endgroup$
    – Jakobian
    Dec 29 '18 at 16:14














4












4








4





$begingroup$

If $xge0$ then $bge0$, hence $$|x|=xle b=|b|lemax(|a|,|b|).$$Similarly if $x<0$ then $a<0$, hence $$|x|=-xle-a=|a|lemax(|a|,|b|).$$






share|cite|improve this answer











$endgroup$



If $xge0$ then $bge0$, hence $$|x|=xle b=|b|lemax(|a|,|b|).$$Similarly if $x<0$ then $a<0$, hence $$|x|=-xle-a=|a|lemax(|a|,|b|).$$







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 29 '18 at 15:50

























answered Dec 29 '18 at 15:47









David C. UllrichDavid C. Ullrich

59.3k43893




59.3k43893












  • $begingroup$
    Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:50












  • $begingroup$
    @Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
    $endgroup$
    – David C. Ullrich
    Dec 29 '18 at 15:52












  • $begingroup$
    Very straightforward proof +1
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:56










  • $begingroup$
    @DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:56






  • 2




    $begingroup$
    @Rebellos $x$ is fixed
    $endgroup$
    – Jakobian
    Dec 29 '18 at 16:14


















  • $begingroup$
    Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:50












  • $begingroup$
    @Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
    $endgroup$
    – David C. Ullrich
    Dec 29 '18 at 15:52












  • $begingroup$
    Very straightforward proof +1
    $endgroup$
    – Jakobian
    Dec 29 '18 at 15:56










  • $begingroup$
    @DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:56






  • 2




    $begingroup$
    @Rebellos $x$ is fixed
    $endgroup$
    – Jakobian
    Dec 29 '18 at 16:14
















$begingroup$
Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
$endgroup$
– Rebellos
Dec 29 '18 at 15:50






$begingroup$
Can't it be $xin [-5,4]$ for example, which means that $x$ can also be positive and negative as well ?
$endgroup$
– Rebellos
Dec 29 '18 at 15:50














$begingroup$
@Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
$endgroup$
– David C. Ullrich
Dec 29 '18 at 15:52






$begingroup$
@Rebellos Huh???? Saying $xin[-5,4]$ does not say that $x$ is both positive and negative - that's total nonsense.
$endgroup$
– David C. Ullrich
Dec 29 '18 at 15:52














$begingroup$
Very straightforward proof +1
$endgroup$
– Jakobian
Dec 29 '18 at 15:56




$begingroup$
Very straightforward proof +1
$endgroup$
– Jakobian
Dec 29 '18 at 15:56












$begingroup$
@DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
$endgroup$
– Rebellos
Dec 29 '18 at 15:56




$begingroup$
@DavidC.Ullrich You hadn't added the second line when I commented :) Still, it's a correct proof. +1 now.
$endgroup$
– Rebellos
Dec 29 '18 at 15:56




2




2




$begingroup$
@Rebellos $x$ is fixed
$endgroup$
– Jakobian
Dec 29 '18 at 16:14




$begingroup$
@Rebellos $x$ is fixed
$endgroup$
– Jakobian
Dec 29 '18 at 16:14











1












$begingroup$

Since $x in [a,b]$, it is $a<b$ (or even $a leq b$ if you want to be more strict for a trivial case).



If $text{sgn}(a) = text{sgn}(b)=1$, then $max{|a|,|b|} = |b| implies |x| leq max{|a|,|b|}$.



If $text{sgn}(a) = text{sgn}(b)=-1$, then $max{|a|,|b|} = |a| implies |x| leq max{|a|,|b|}$.



In the case of $text{sgn}(a) neq text{sgn}(b)$, that means that $a$ would be negative. But, in that case, it can be $|a| > |b|$. Then, for $|x|$, it would be : $|x| leq |a| implies |x| leq max{|a|,|b|}$.



So, for all cases, we have that : $|x| leq max{|a|,|b|}$.



Edit (Alternativelly) : The comment of Jakobian is really nice to consider for a really straightforward proof. What the absolute value $|x|$ means is the distance of $x$ from $0$. But since $x in [a,b]$, then this distance cannot be larger than the distance of $a$ from zero $(|a|)$ or the distance of $b$ from zero $(|b|)$.






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
    $endgroup$
    – Noble Mushtak
    Dec 29 '18 at 15:45










  • $begingroup$
    @NobleMushtak Sure, edited. I missed a line.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:46


















1












$begingroup$

Since $x in [a,b]$, it is $a<b$ (or even $a leq b$ if you want to be more strict for a trivial case).



If $text{sgn}(a) = text{sgn}(b)=1$, then $max{|a|,|b|} = |b| implies |x| leq max{|a|,|b|}$.



If $text{sgn}(a) = text{sgn}(b)=-1$, then $max{|a|,|b|} = |a| implies |x| leq max{|a|,|b|}$.



In the case of $text{sgn}(a) neq text{sgn}(b)$, that means that $a$ would be negative. But, in that case, it can be $|a| > |b|$. Then, for $|x|$, it would be : $|x| leq |a| implies |x| leq max{|a|,|b|}$.



So, for all cases, we have that : $|x| leq max{|a|,|b|}$.



Edit (Alternativelly) : The comment of Jakobian is really nice to consider for a really straightforward proof. What the absolute value $|x|$ means is the distance of $x$ from $0$. But since $x in [a,b]$, then this distance cannot be larger than the distance of $a$ from zero $(|a|)$ or the distance of $b$ from zero $(|b|)$.






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
    $endgroup$
    – Noble Mushtak
    Dec 29 '18 at 15:45










  • $begingroup$
    @NobleMushtak Sure, edited. I missed a line.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:46
















1












1








1





$begingroup$

Since $x in [a,b]$, it is $a<b$ (or even $a leq b$ if you want to be more strict for a trivial case).



If $text{sgn}(a) = text{sgn}(b)=1$, then $max{|a|,|b|} = |b| implies |x| leq max{|a|,|b|}$.



If $text{sgn}(a) = text{sgn}(b)=-1$, then $max{|a|,|b|} = |a| implies |x| leq max{|a|,|b|}$.



In the case of $text{sgn}(a) neq text{sgn}(b)$, that means that $a$ would be negative. But, in that case, it can be $|a| > |b|$. Then, for $|x|$, it would be : $|x| leq |a| implies |x| leq max{|a|,|b|}$.



So, for all cases, we have that : $|x| leq max{|a|,|b|}$.



Edit (Alternativelly) : The comment of Jakobian is really nice to consider for a really straightforward proof. What the absolute value $|x|$ means is the distance of $x$ from $0$. But since $x in [a,b]$, then this distance cannot be larger than the distance of $a$ from zero $(|a|)$ or the distance of $b$ from zero $(|b|)$.






share|cite|improve this answer











$endgroup$



Since $x in [a,b]$, it is $a<b$ (or even $a leq b$ if you want to be more strict for a trivial case).



If $text{sgn}(a) = text{sgn}(b)=1$, then $max{|a|,|b|} = |b| implies |x| leq max{|a|,|b|}$.



If $text{sgn}(a) = text{sgn}(b)=-1$, then $max{|a|,|b|} = |a| implies |x| leq max{|a|,|b|}$.



In the case of $text{sgn}(a) neq text{sgn}(b)$, that means that $a$ would be negative. But, in that case, it can be $|a| > |b|$. Then, for $|x|$, it would be : $|x| leq |a| implies |x| leq max{|a|,|b|}$.



So, for all cases, we have that : $|x| leq max{|a|,|b|}$.



Edit (Alternativelly) : The comment of Jakobian is really nice to consider for a really straightforward proof. What the absolute value $|x|$ means is the distance of $x$ from $0$. But since $x in [a,b]$, then this distance cannot be larger than the distance of $a$ from zero $(|a|)$ or the distance of $b$ from zero $(|b|)$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 29 '18 at 15:54

























answered Dec 29 '18 at 15:43









RebellosRebellos

14.5k31246




14.5k31246








  • 1




    $begingroup$
    I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
    $endgroup$
    – Noble Mushtak
    Dec 29 '18 at 15:45










  • $begingroup$
    @NobleMushtak Sure, edited. I missed a line.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:46
















  • 1




    $begingroup$
    I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
    $endgroup$
    – Noble Mushtak
    Dec 29 '18 at 15:45










  • $begingroup$
    @NobleMushtak Sure, edited. I missed a line.
    $endgroup$
    – Rebellos
    Dec 29 '18 at 15:46










1




1




$begingroup$
I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
$endgroup$
– Noble Mushtak
Dec 29 '18 at 15:45




$begingroup$
I do not understand your first statement. If I have $a=-3$ and $b=-2$, then the maximum of $|a|$ and $|b|$ would be $|a|=3$, not $|b|=2$.
$endgroup$
– Noble Mushtak
Dec 29 '18 at 15:45












$begingroup$
@NobleMushtak Sure, edited. I missed a line.
$endgroup$
– Rebellos
Dec 29 '18 at 15:46






$begingroup$
@NobleMushtak Sure, edited. I missed a line.
$endgroup$
– Rebellos
Dec 29 '18 at 15:46




















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