Proving $frac{a}{a^2-a+1}+frac{b}{b^2-b+1}+frac{c}{c^2-c+1}+frac{d}{d^2-d+1}le frac{8}{3}$ given $a+b+c+d=2$












3












$begingroup$



Let $a,b,c,din mathbb{R}$ and $a+b+c+d=2$. Prove that $$frac{a}{a^2-a+1}+frac{b}{b^2-b+1}+frac{c}{c^2-c+1}+frac{d}{d^2-d+1}le frac{8}{3}.$$




We have $$frac{a}{a^2-a+1}le frac{4}{3}aLongleftrightarrow frac{-aleft(2a-1right)^2}{3left(a^2-a+1right)}le 0,$$ (Right)



So $$LHSle frac{4}{3}left(a+b+c+dright)=frac{8}{3}=RHS$$



When $a=b=c=d=frac{1}{2}$



But also $a=b=c=1;d=-1$ i don't know how to solve it.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    What have you attempted thus far?
    $endgroup$
    – user477343
    Feb 27 '18 at 11:45










  • $begingroup$
    @Word Please mention what you have already tried.
    $endgroup$
    – Fitz Watson
    Feb 27 '18 at 12:20










  • $begingroup$
    $$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
    $endgroup$
    – Amin235
    Feb 27 '18 at 12:34






  • 1




    $begingroup$
    This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
    $endgroup$
    – Carl Mummert
    Feb 27 '18 at 13:19
















3












$begingroup$



Let $a,b,c,din mathbb{R}$ and $a+b+c+d=2$. Prove that $$frac{a}{a^2-a+1}+frac{b}{b^2-b+1}+frac{c}{c^2-c+1}+frac{d}{d^2-d+1}le frac{8}{3}.$$




We have $$frac{a}{a^2-a+1}le frac{4}{3}aLongleftrightarrow frac{-aleft(2a-1right)^2}{3left(a^2-a+1right)}le 0,$$ (Right)



So $$LHSle frac{4}{3}left(a+b+c+dright)=frac{8}{3}=RHS$$



When $a=b=c=d=frac{1}{2}$



But also $a=b=c=1;d=-1$ i don't know how to solve it.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    What have you attempted thus far?
    $endgroup$
    – user477343
    Feb 27 '18 at 11:45










  • $begingroup$
    @Word Please mention what you have already tried.
    $endgroup$
    – Fitz Watson
    Feb 27 '18 at 12:20










  • $begingroup$
    $$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
    $endgroup$
    – Amin235
    Feb 27 '18 at 12:34






  • 1




    $begingroup$
    This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
    $endgroup$
    – Carl Mummert
    Feb 27 '18 at 13:19














3












3








3


2



$begingroup$



Let $a,b,c,din mathbb{R}$ and $a+b+c+d=2$. Prove that $$frac{a}{a^2-a+1}+frac{b}{b^2-b+1}+frac{c}{c^2-c+1}+frac{d}{d^2-d+1}le frac{8}{3}.$$




We have $$frac{a}{a^2-a+1}le frac{4}{3}aLongleftrightarrow frac{-aleft(2a-1right)^2}{3left(a^2-a+1right)}le 0,$$ (Right)



So $$LHSle frac{4}{3}left(a+b+c+dright)=frac{8}{3}=RHS$$



When $a=b=c=d=frac{1}{2}$



But also $a=b=c=1;d=-1$ i don't know how to solve it.










share|cite|improve this question











$endgroup$





Let $a,b,c,din mathbb{R}$ and $a+b+c+d=2$. Prove that $$frac{a}{a^2-a+1}+frac{b}{b^2-b+1}+frac{c}{c^2-c+1}+frac{d}{d^2-d+1}le frac{8}{3}.$$




We have $$frac{a}{a^2-a+1}le frac{4}{3}aLongleftrightarrow frac{-aleft(2a-1right)^2}{3left(a^2-a+1right)}le 0,$$ (Right)



So $$LHSle frac{4}{3}left(a+b+c+dright)=frac{8}{3}=RHS$$



When $a=b=c=d=frac{1}{2}$



But also $a=b=c=1;d=-1$ i don't know how to solve it.







inequality






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 29 at 11:48







Word Shallow

















asked Feb 27 '18 at 11:39









Word ShallowWord Shallow

1,0832622




1,0832622








  • 1




    $begingroup$
    What have you attempted thus far?
    $endgroup$
    – user477343
    Feb 27 '18 at 11:45










  • $begingroup$
    @Word Please mention what you have already tried.
    $endgroup$
    – Fitz Watson
    Feb 27 '18 at 12:20










  • $begingroup$
    $$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
    $endgroup$
    – Amin235
    Feb 27 '18 at 12:34






  • 1




    $begingroup$
    This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
    $endgroup$
    – Carl Mummert
    Feb 27 '18 at 13:19














  • 1




    $begingroup$
    What have you attempted thus far?
    $endgroup$
    – user477343
    Feb 27 '18 at 11:45










  • $begingroup$
    @Word Please mention what you have already tried.
    $endgroup$
    – Fitz Watson
    Feb 27 '18 at 12:20










  • $begingroup$
    $$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
    $endgroup$
    – Amin235
    Feb 27 '18 at 12:34






  • 1




    $begingroup$
    This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
    $endgroup$
    – Carl Mummert
    Feb 27 '18 at 13:19








1




1




$begingroup$
What have you attempted thus far?
$endgroup$
– user477343
Feb 27 '18 at 11:45




$begingroup$
What have you attempted thus far?
$endgroup$
– user477343
Feb 27 '18 at 11:45












$begingroup$
@Word Please mention what you have already tried.
$endgroup$
– Fitz Watson
Feb 27 '18 at 12:20




$begingroup$
@Word Please mention what you have already tried.
$endgroup$
– Fitz Watson
Feb 27 '18 at 12:20












$begingroup$
$$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
$endgroup$
– Amin235
Feb 27 '18 at 12:34




$begingroup$
$$ f(a,b,c,d)=frac{1}{frac{(a-1)^2}{a}+1}+frac{1}{frac{(b-1)^2}{b}+1}+frac{1}{frac{(c-1)^2}{c}+1}+frac{1}{frac{(d-1)^2}{d}+1} $$
$endgroup$
– Amin235
Feb 27 '18 at 12:34




1




1




$begingroup$
This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
$endgroup$
– Carl Mummert
Feb 27 '18 at 13:19




$begingroup$
This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem, its source, the motivation behind the inequality, and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
$endgroup$
– Carl Mummert
Feb 27 '18 at 13:19










1 Answer
1






active

oldest

votes


















3












$begingroup$

First, we simplify a bit by transforming $x = a-frac12, y = b - frac12, z = c - frac12, w = d - frac12$, so that we need to prove equivalently with $x+y+z+w=0$, the inequality
$$sum frac{2x+1}{4x^2+3} leqslant frac43$$



Here $sum$ represents cyclic sums. Also as $displaystyle 1 - 2cdotfrac{2x+1}{4x^2+3} = frac{(2x-1)^2}{4x^2+3}$, we may equivalently show
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac43$$



Now we need an upper bound on the denominator; for this, note



$$4x^2+3=3x^2+(y+z+w)^2+3leqslant 3x^2+3(y^2+z^2+w^2)+3 = 3sum x^2+3$$
Thus we have a common denominator now, therefore,
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac{sum (2x-1)^2}{3sum x^2+3} = frac{4sum x^2-4sum x+4}{3(sum x^2+1)}=frac43$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:07










  • $begingroup$
    but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:09










  • $begingroup$
    Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
    $endgroup$
    – Macavity
    Mar 9 '18 at 12:27














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1 Answer
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active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

First, we simplify a bit by transforming $x = a-frac12, y = b - frac12, z = c - frac12, w = d - frac12$, so that we need to prove equivalently with $x+y+z+w=0$, the inequality
$$sum frac{2x+1}{4x^2+3} leqslant frac43$$



Here $sum$ represents cyclic sums. Also as $displaystyle 1 - 2cdotfrac{2x+1}{4x^2+3} = frac{(2x-1)^2}{4x^2+3}$, we may equivalently show
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac43$$



Now we need an upper bound on the denominator; for this, note



$$4x^2+3=3x^2+(y+z+w)^2+3leqslant 3x^2+3(y^2+z^2+w^2)+3 = 3sum x^2+3$$
Thus we have a common denominator now, therefore,
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac{sum (2x-1)^2}{3sum x^2+3} = frac{4sum x^2-4sum x+4}{3(sum x^2+1)}=frac43$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:07










  • $begingroup$
    but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:09










  • $begingroup$
    Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
    $endgroup$
    – Macavity
    Mar 9 '18 at 12:27


















3












$begingroup$

First, we simplify a bit by transforming $x = a-frac12, y = b - frac12, z = c - frac12, w = d - frac12$, so that we need to prove equivalently with $x+y+z+w=0$, the inequality
$$sum frac{2x+1}{4x^2+3} leqslant frac43$$



Here $sum$ represents cyclic sums. Also as $displaystyle 1 - 2cdotfrac{2x+1}{4x^2+3} = frac{(2x-1)^2}{4x^2+3}$, we may equivalently show
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac43$$



Now we need an upper bound on the denominator; for this, note



$$4x^2+3=3x^2+(y+z+w)^2+3leqslant 3x^2+3(y^2+z^2+w^2)+3 = 3sum x^2+3$$
Thus we have a common denominator now, therefore,
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac{sum (2x-1)^2}{3sum x^2+3} = frac{4sum x^2-4sum x+4}{3(sum x^2+1)}=frac43$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:07










  • $begingroup$
    but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:09










  • $begingroup$
    Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
    $endgroup$
    – Macavity
    Mar 9 '18 at 12:27
















3












3








3





$begingroup$

First, we simplify a bit by transforming $x = a-frac12, y = b - frac12, z = c - frac12, w = d - frac12$, so that we need to prove equivalently with $x+y+z+w=0$, the inequality
$$sum frac{2x+1}{4x^2+3} leqslant frac43$$



Here $sum$ represents cyclic sums. Also as $displaystyle 1 - 2cdotfrac{2x+1}{4x^2+3} = frac{(2x-1)^2}{4x^2+3}$, we may equivalently show
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac43$$



Now we need an upper bound on the denominator; for this, note



$$4x^2+3=3x^2+(y+z+w)^2+3leqslant 3x^2+3(y^2+z^2+w^2)+3 = 3sum x^2+3$$
Thus we have a common denominator now, therefore,
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac{sum (2x-1)^2}{3sum x^2+3} = frac{4sum x^2-4sum x+4}{3(sum x^2+1)}=frac43$$






share|cite|improve this answer









$endgroup$



First, we simplify a bit by transforming $x = a-frac12, y = b - frac12, z = c - frac12, w = d - frac12$, so that we need to prove equivalently with $x+y+z+w=0$, the inequality
$$sum frac{2x+1}{4x^2+3} leqslant frac43$$



Here $sum$ represents cyclic sums. Also as $displaystyle 1 - 2cdotfrac{2x+1}{4x^2+3} = frac{(2x-1)^2}{4x^2+3}$, we may equivalently show
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac43$$



Now we need an upper bound on the denominator; for this, note



$$4x^2+3=3x^2+(y+z+w)^2+3leqslant 3x^2+3(y^2+z^2+w^2)+3 = 3sum x^2+3$$
Thus we have a common denominator now, therefore,
$$sum frac{(2x-1)^2}{4x^2+3} geqslant frac{sum (2x-1)^2}{3sum x^2+3} = frac{4sum x^2-4sum x+4}{3(sum x^2+1)}=frac43$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Feb 27 '18 at 17:42









MacavityMacavity

35.7k52554




35.7k52554












  • $begingroup$
    $(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:07










  • $begingroup$
    but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:09










  • $begingroup$
    Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
    $endgroup$
    – Macavity
    Mar 9 '18 at 12:27




















  • $begingroup$
    $(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:07










  • $begingroup$
    but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
    $endgroup$
    – Word Shallow
    Mar 9 '18 at 12:09










  • $begingroup$
    Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
    $endgroup$
    – Macavity
    Mar 9 '18 at 12:27


















$begingroup$
$(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
$endgroup$
– Word Shallow
Mar 9 '18 at 12:07




$begingroup$
$(y+z+w)^2le 3left(y^2+z^2+w^2right)Leftrightarrow y=z=w$ So we have $z=x=w,x=y=w,x=y=zRightarrow x=y=z=w=1$
$endgroup$
– Word Shallow
Mar 9 '18 at 12:07












$begingroup$
but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
$endgroup$
– Word Shallow
Mar 9 '18 at 12:09




$begingroup$
but in that case, the equaLity occur when a=b=c=1,d=-1 and i can't soIve it
$endgroup$
– Word Shallow
Mar 9 '18 at 12:09












$begingroup$
Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
$endgroup$
– Macavity
Mar 9 '18 at 12:27






$begingroup$
Note the inequality is proven, you are looking for equality cases. In terms of $x,y,z,w$ equality is when $x=y=z=w=0$ and when $x,y,z=frac12, w=-frac32$ or permutations. Note in the latter case, numerators become zero, so the bounds on denominators are irrelevant.
$endgroup$
– Macavity
Mar 9 '18 at 12:27




















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