Proving a three variables inequality
up vote
3
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Given that $frac{1}{a}+frac{1}{b}+frac{1}{c}=1$ show that:$$(a+1)(b+1)(c+1)ge 64$$
My attempt: First I tried expanding the LHS getting that$$abc+ab+bc+ca+a+b+c ge 63$$
I applied Cauchy-Schwarz on $(a+b+c)(frac{1}{a}+frac{1}{b}+frac{1}{c})$ getting that $a+b+cge9$.
Then I also tried to manipulate the first condition and got $abc=ab+bc+ca$, then I applied AM-GM on $a+b+c$ getting the following$$a+b+cge3sqrt[3]{abc}$$
Finally I substituted $ab+bc+ca=abc$ in my initial expression, getting:$$2abc+(a+b+c)ge63$$The last thing I tought about was that I have both $a+b+cge9$ and $a+b+cge3sqrt[3]{abc}$ so if I somehow related them I would have $sqrt[3]{abc} ge 3 rightarrow abcge27$ and with this conditions the problem would follow by summing, but the direction of the inequality is not allowing me to do as intended...
I'm stuck here, have tried lot of other things but nothing really worked, also partial help is appreciated!
inequality
add a comment |
up vote
3
down vote
favorite
Given that $frac{1}{a}+frac{1}{b}+frac{1}{c}=1$ show that:$$(a+1)(b+1)(c+1)ge 64$$
My attempt: First I tried expanding the LHS getting that$$abc+ab+bc+ca+a+b+c ge 63$$
I applied Cauchy-Schwarz on $(a+b+c)(frac{1}{a}+frac{1}{b}+frac{1}{c})$ getting that $a+b+cge9$.
Then I also tried to manipulate the first condition and got $abc=ab+bc+ca$, then I applied AM-GM on $a+b+c$ getting the following$$a+b+cge3sqrt[3]{abc}$$
Finally I substituted $ab+bc+ca=abc$ in my initial expression, getting:$$2abc+(a+b+c)ge63$$The last thing I tought about was that I have both $a+b+cge9$ and $a+b+cge3sqrt[3]{abc}$ so if I somehow related them I would have $sqrt[3]{abc} ge 3 rightarrow abcge27$ and with this conditions the problem would follow by summing, but the direction of the inequality is not allowing me to do as intended...
I'm stuck here, have tried lot of other things but nothing really worked, also partial help is appreciated!
inequality
I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago
add a comment |
up vote
3
down vote
favorite
up vote
3
down vote
favorite
Given that $frac{1}{a}+frac{1}{b}+frac{1}{c}=1$ show that:$$(a+1)(b+1)(c+1)ge 64$$
My attempt: First I tried expanding the LHS getting that$$abc+ab+bc+ca+a+b+c ge 63$$
I applied Cauchy-Schwarz on $(a+b+c)(frac{1}{a}+frac{1}{b}+frac{1}{c})$ getting that $a+b+cge9$.
Then I also tried to manipulate the first condition and got $abc=ab+bc+ca$, then I applied AM-GM on $a+b+c$ getting the following$$a+b+cge3sqrt[3]{abc}$$
Finally I substituted $ab+bc+ca=abc$ in my initial expression, getting:$$2abc+(a+b+c)ge63$$The last thing I tought about was that I have both $a+b+cge9$ and $a+b+cge3sqrt[3]{abc}$ so if I somehow related them I would have $sqrt[3]{abc} ge 3 rightarrow abcge27$ and with this conditions the problem would follow by summing, but the direction of the inequality is not allowing me to do as intended...
I'm stuck here, have tried lot of other things but nothing really worked, also partial help is appreciated!
inequality
Given that $frac{1}{a}+frac{1}{b}+frac{1}{c}=1$ show that:$$(a+1)(b+1)(c+1)ge 64$$
My attempt: First I tried expanding the LHS getting that$$abc+ab+bc+ca+a+b+c ge 63$$
I applied Cauchy-Schwarz on $(a+b+c)(frac{1}{a}+frac{1}{b}+frac{1}{c})$ getting that $a+b+cge9$.
Then I also tried to manipulate the first condition and got $abc=ab+bc+ca$, then I applied AM-GM on $a+b+c$ getting the following$$a+b+cge3sqrt[3]{abc}$$
Finally I substituted $ab+bc+ca=abc$ in my initial expression, getting:$$2abc+(a+b+c)ge63$$The last thing I tought about was that I have both $a+b+cge9$ and $a+b+cge3sqrt[3]{abc}$ so if I somehow related them I would have $sqrt[3]{abc} ge 3 rightarrow abcge27$ and with this conditions the problem would follow by summing, but the direction of the inequality is not allowing me to do as intended...
I'm stuck here, have tried lot of other things but nothing really worked, also partial help is appreciated!
inequality
inequality
asked 2 hours ago
Spasoje Durovic
1218
1218
I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago
add a comment |
I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago
I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago
I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago
add a comment |
4 Answers
4
active
oldest
votes
up vote
7
down vote
accepted
By the super-additivity of the geometric mean / convexity of $log(e^x+1)$
$$ (1+a)(1+b)(1+c) geq (1+sqrt[3]{abc})^3 $$
and by the GM-HM inequality
$$ sqrt[3]{abc} geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}=3.$$
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
|
show 8 more comments
up vote
5
down vote
I think you mean $$a,b,c>0$$ in this case we have
$$frac{a+b+c}{3}geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}$$ so $$a+b+cgeq 9$$ and we get also $$frac{bc+ac+ab}{3}geq sqrt[3]{(abc)^2}$$ so $$abcgeq 27$$ and $$ab+ac+bcgeq 27$$ putting things together we have
$$(a+1)(b+1)(c+1)=abc+ab+bc+ca+a+b+c+1geq 27+27+10=64$$
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
add a comment |
up vote
3
down vote
Since $ab+bc+ca= abc$ and $$ab+bc+cageq 3sqrt[3]{a^2b^2c^2}implies abcgeq 27$$
Now $$(a+1)(b+1)(c+1)=2abc+a+b+c+1geq 55+3sqrt[3]{27} = 64$$
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
Never mind, it is OK
– greedoid
1 hour ago
add a comment |
up vote
1
down vote
We need to prove that
$$sum_{cyc}(ln(1+a)-2ln2)geq0$$ or
$$sum_{cyc}left(ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)right)geq0,$$
which is true because
$$f(a)=ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)geq0$$ for all $a>0$.
Indeed, $$f'(a)=frac{(a-3)(4a+3)}{4a^2(a+1)},$$ which gives $a_{min}=3$ and since $f(3)=0$, we are done!
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
add a comment |
4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
7
down vote
accepted
By the super-additivity of the geometric mean / convexity of $log(e^x+1)$
$$ (1+a)(1+b)(1+c) geq (1+sqrt[3]{abc})^3 $$
and by the GM-HM inequality
$$ sqrt[3]{abc} geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}=3.$$
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
|
show 8 more comments
up vote
7
down vote
accepted
By the super-additivity of the geometric mean / convexity of $log(e^x+1)$
$$ (1+a)(1+b)(1+c) geq (1+sqrt[3]{abc})^3 $$
and by the GM-HM inequality
$$ sqrt[3]{abc} geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}=3.$$
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
|
show 8 more comments
up vote
7
down vote
accepted
up vote
7
down vote
accepted
By the super-additivity of the geometric mean / convexity of $log(e^x+1)$
$$ (1+a)(1+b)(1+c) geq (1+sqrt[3]{abc})^3 $$
and by the GM-HM inequality
$$ sqrt[3]{abc} geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}=3.$$
By the super-additivity of the geometric mean / convexity of $log(e^x+1)$
$$ (1+a)(1+b)(1+c) geq (1+sqrt[3]{abc})^3 $$
and by the GM-HM inequality
$$ sqrt[3]{abc} geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}=3.$$
answered 2 hours ago
Jack D'Aurizio
284k33275653
284k33275653
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
|
show 8 more comments
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
1
1
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
Please explain the downvote. I do not see anything wrong with this approach, it is pretty straightforward.
– Jack D'Aurizio
2 hours ago
3
3
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
Maybe the downvoter's arithmetic is so bad that they want you explain that $4^3 = 64$ $ddot{smile}$.
– Rob Arthan
2 hours ago
1
1
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
@JackD'Aurizio I'm convinced that there are users who stays here on MSE only to downvote, probably to give vent to frustration in the real world!
– gimusi
2 hours ago
1
1
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
@greedoid Nothing personal with the comment, I was only joking of course.
– gimusi
2 hours ago
1
1
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
@RobArthan Not a theory but only a joke! Bye :)
– gimusi
1 hour ago
|
show 8 more comments
up vote
5
down vote
I think you mean $$a,b,c>0$$ in this case we have
$$frac{a+b+c}{3}geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}$$ so $$a+b+cgeq 9$$ and we get also $$frac{bc+ac+ab}{3}geq sqrt[3]{(abc)^2}$$ so $$abcgeq 27$$ and $$ab+ac+bcgeq 27$$ putting things together we have
$$(a+1)(b+1)(c+1)=abc+ab+bc+ca+a+b+c+1geq 27+27+10=64$$
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
add a comment |
up vote
5
down vote
I think you mean $$a,b,c>0$$ in this case we have
$$frac{a+b+c}{3}geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}$$ so $$a+b+cgeq 9$$ and we get also $$frac{bc+ac+ab}{3}geq sqrt[3]{(abc)^2}$$ so $$abcgeq 27$$ and $$ab+ac+bcgeq 27$$ putting things together we have
$$(a+1)(b+1)(c+1)=abc+ab+bc+ca+a+b+c+1geq 27+27+10=64$$
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
add a comment |
up vote
5
down vote
up vote
5
down vote
I think you mean $$a,b,c>0$$ in this case we have
$$frac{a+b+c}{3}geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}$$ so $$a+b+cgeq 9$$ and we get also $$frac{bc+ac+ab}{3}geq sqrt[3]{(abc)^2}$$ so $$abcgeq 27$$ and $$ab+ac+bcgeq 27$$ putting things together we have
$$(a+1)(b+1)(c+1)=abc+ab+bc+ca+a+b+c+1geq 27+27+10=64$$
I think you mean $$a,b,c>0$$ in this case we have
$$frac{a+b+c}{3}geq frac{3}{frac{1}{a}+frac{1}{b}+frac{1}{c}}$$ so $$a+b+cgeq 9$$ and we get also $$frac{bc+ac+ab}{3}geq sqrt[3]{(abc)^2}$$ so $$abcgeq 27$$ and $$ab+ac+bcgeq 27$$ putting things together we have
$$(a+1)(b+1)(c+1)=abc+ab+bc+ca+a+b+c+1geq 27+27+10=64$$
answered 2 hours ago
Dr. Sonnhard Graubner
71.6k32864
71.6k32864
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
add a comment |
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Welcome back Dr. Graubner!
– gimusi
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Yeah, OP forgot to mention that $a,b,c$ have to be positive, I forgot to add that I had noticed that
– Spasoje Durovic
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
Thanks, i'm also glad to see you!
– Dr. Sonnhard Graubner
1 hour ago
add a comment |
up vote
3
down vote
Since $ab+bc+ca= abc$ and $$ab+bc+cageq 3sqrt[3]{a^2b^2c^2}implies abcgeq 27$$
Now $$(a+1)(b+1)(c+1)=2abc+a+b+c+1geq 55+3sqrt[3]{27} = 64$$
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
Never mind, it is OK
– greedoid
1 hour ago
add a comment |
up vote
3
down vote
Since $ab+bc+ca= abc$ and $$ab+bc+cageq 3sqrt[3]{a^2b^2c^2}implies abcgeq 27$$
Now $$(a+1)(b+1)(c+1)=2abc+a+b+c+1geq 55+3sqrt[3]{27} = 64$$
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
Never mind, it is OK
– greedoid
1 hour ago
add a comment |
up vote
3
down vote
up vote
3
down vote
Since $ab+bc+ca= abc$ and $$ab+bc+cageq 3sqrt[3]{a^2b^2c^2}implies abcgeq 27$$
Now $$(a+1)(b+1)(c+1)=2abc+a+b+c+1geq 55+3sqrt[3]{27} = 64$$
Since $ab+bc+ca= abc$ and $$ab+bc+cageq 3sqrt[3]{a^2b^2c^2}implies abcgeq 27$$
Now $$(a+1)(b+1)(c+1)=2abc+a+b+c+1geq 55+3sqrt[3]{27} = 64$$
answered 2 hours ago
greedoid
35.8k114590
35.8k114590
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
Never mind, it is OK
– greedoid
1 hour ago
add a comment |
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
Never mind, it is OK
– greedoid
1 hour ago
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yup, that first passage was the one I didn't see, I'm actually terrible at AM-GM!...
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Yeah, I've started working on inequalities about a week ago, hope I'll get better soon! cheers (nice to see you're from the Balkans too :P)
– Spasoje Durovic
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
Actualy I'm not.
– greedoid
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
My bad! I supposed it since you used Cyrillic
– Spasoje Durovic
1 hour ago
1
1
Never mind, it is OK
– greedoid
1 hour ago
Never mind, it is OK
– greedoid
1 hour ago
add a comment |
up vote
1
down vote
We need to prove that
$$sum_{cyc}(ln(1+a)-2ln2)geq0$$ or
$$sum_{cyc}left(ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)right)geq0,$$
which is true because
$$f(a)=ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)geq0$$ for all $a>0$.
Indeed, $$f'(a)=frac{(a-3)(4a+3)}{4a^2(a+1)},$$ which gives $a_{min}=3$ and since $f(3)=0$, we are done!
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
add a comment |
up vote
1
down vote
We need to prove that
$$sum_{cyc}(ln(1+a)-2ln2)geq0$$ or
$$sum_{cyc}left(ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)right)geq0,$$
which is true because
$$f(a)=ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)geq0$$ for all $a>0$.
Indeed, $$f'(a)=frac{(a-3)(4a+3)}{4a^2(a+1)},$$ which gives $a_{min}=3$ and since $f(3)=0$, we are done!
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
add a comment |
up vote
1
down vote
up vote
1
down vote
We need to prove that
$$sum_{cyc}(ln(1+a)-2ln2)geq0$$ or
$$sum_{cyc}left(ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)right)geq0,$$
which is true because
$$f(a)=ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)geq0$$ for all $a>0$.
Indeed, $$f'(a)=frac{(a-3)(4a+3)}{4a^2(a+1)},$$ which gives $a_{min}=3$ and since $f(3)=0$, we are done!
We need to prove that
$$sum_{cyc}(ln(1+a)-2ln2)geq0$$ or
$$sum_{cyc}left(ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)right)geq0,$$
which is true because
$$f(a)=ln(1+a)-2ln2+frac{9}{4}left(frac{1}{a}-frac{1}{3}right)geq0$$ for all $a>0$.
Indeed, $$f'(a)=frac{(a-3)(4a+3)}{4a^2(a+1)},$$ which gives $a_{min}=3$ and since $f(3)=0$, we are done!
answered 1 hour ago
Michael Rozenberg
94.6k1588183
94.6k1588183
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
add a comment |
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
Can you elaborate on how you got the first expression?
– YiFan
25 mins ago
add a comment |
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I haven't tried this I admit .... but my instant thought is, since we 'know' (please note the apostrophæ!) that it attains its minimum value at a=b=c=3, could we not hypothesise that it's 3, plug in 3+δ, 3+ε, & 3+ζ, & show that any increase in some measure of the size of δ, ε & ζ causes an increase in the product under the constraint? As I said - just my instantaneous thought on seeing the question!
– AmbretteOrrisey
24 mins ago