Minimum $M=frac{4}{x^{2}}+frac{4}{y^{2}}+frac{1}{(x-y)^{2}}$ for $3x^2+8y^3=20$
$begingroup$
Let $x,y$ be positive real numbers such that $xneq y$ and
$3x^{2}+8y^{3}=20$. Find $x,y$ so that $M$ get the minimum value:
$M=frac{4}{x^{2}}+frac{4}{y^{2}}+frac{1}{(x-y)^{2}}$.
I can not find when this function gets the minimum value. Or can this difficult problem be solved by differentiating? How do we turn the root to another specie which has $ax+by$ or $frac{ax}{y}$ while $x,y$ are some constants.
Help me to solve this problem.
calculus inequality optimization nonlinear-optimization maxima-minima
$endgroup$
add a comment |
$begingroup$
Let $x,y$ be positive real numbers such that $xneq y$ and
$3x^{2}+8y^{3}=20$. Find $x,y$ so that $M$ get the minimum value:
$M=frac{4}{x^{2}}+frac{4}{y^{2}}+frac{1}{(x-y)^{2}}$.
I can not find when this function gets the minimum value. Or can this difficult problem be solved by differentiating? How do we turn the root to another specie which has $ax+by$ or $frac{ax}{y}$ while $x,y$ are some constants.
Help me to solve this problem.
calculus inequality optimization nonlinear-optimization maxima-minima
$endgroup$
1
$begingroup$
What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55
add a comment |
$begingroup$
Let $x,y$ be positive real numbers such that $xneq y$ and
$3x^{2}+8y^{3}=20$. Find $x,y$ so that $M$ get the minimum value:
$M=frac{4}{x^{2}}+frac{4}{y^{2}}+frac{1}{(x-y)^{2}}$.
I can not find when this function gets the minimum value. Or can this difficult problem be solved by differentiating? How do we turn the root to another specie which has $ax+by$ or $frac{ax}{y}$ while $x,y$ are some constants.
Help me to solve this problem.
calculus inequality optimization nonlinear-optimization maxima-minima
$endgroup$
Let $x,y$ be positive real numbers such that $xneq y$ and
$3x^{2}+8y^{3}=20$. Find $x,y$ so that $M$ get the minimum value:
$M=frac{4}{x^{2}}+frac{4}{y^{2}}+frac{1}{(x-y)^{2}}$.
I can not find when this function gets the minimum value. Or can this difficult problem be solved by differentiating? How do we turn the root to another specie which has $ax+by$ or $frac{ax}{y}$ while $x,y$ are some constants.
Help me to solve this problem.
calculus inequality optimization nonlinear-optimization maxima-minima
calculus inequality optimization nonlinear-optimization maxima-minima
edited Dec 13 '18 at 14:27
Martin Sleziak
44.8k10119272
44.8k10119272
asked Dec 13 '18 at 11:26
Trong TuanTrong Tuan
1318
1318
1
$begingroup$
What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55
add a comment |
1
$begingroup$
What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55
1
1
$begingroup$
What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55
$begingroup$
What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Let $x=2$ and $y=1$.
Thus, we'll get a value $6$.
We'll prove that it's a minimal value.
Indeed, let $x=2a$ and $y=b$.
Thus, $3a^2+2b^3=5$ and we need to prove that
$$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geq6.$$
But by AM-GM
$$5=3a^2+2b^3=3a^2-1+3b^2+2b^3+1-3b^2geq$$
$$geq3a^2+3b^2-1+3sqrt[3]{(b^3)^2cdot1}-3b^2=3(a^2+b^2)-1,$$
which gives $2geq a^2+b^2.$
Id est, $$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geqfrac{1}{2}(a^2+b^2)left(frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}right).$$
Now, let $a=tb$.
Thus, we need to prove that
$$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ or
$$(t-1)^2(16t^4+16t^3-7t^2-2t+1)geq0,$$ which is obviously true for $tgeq1.$
But for $0<tleq1$ we obtain:
$$16t^4+16t^3-7t^2-2t+1=$$
$$=9t^3(1-t)+left(5t^2+frac{7}{10}t-frac{5}{6}right)^2+frac{1}{900}(759t^2-750t+275)geq0.$$
Done!
Also, we can use derivative here.
Since $$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ it's $f(t)geq0,$ where
$$f(t)=64t^6-64t^5-92t^4+112t^3-8t^2-16t+4,$$ and
$$f'(t)=(t-1)(t+1)(3t-1)(8t^2-4t-1),$$
we obtain $t_{min}=frac{1}{3}$ or $t_{min}=1$ and since $fleft(frac{1}{3}right)>f(1)=0$, we are done.
$endgroup$
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
add a comment |
$begingroup$
Local minimum:
$$f(2,1)=6$$
Global minimum:
$$f(−1.668453756732446,1.133435720989714)=4.677920159716602$$
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
Let $x=2$ and $y=1$.
Thus, we'll get a value $6$.
We'll prove that it's a minimal value.
Indeed, let $x=2a$ and $y=b$.
Thus, $3a^2+2b^3=5$ and we need to prove that
$$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geq6.$$
But by AM-GM
$$5=3a^2+2b^3=3a^2-1+3b^2+2b^3+1-3b^2geq$$
$$geq3a^2+3b^2-1+3sqrt[3]{(b^3)^2cdot1}-3b^2=3(a^2+b^2)-1,$$
which gives $2geq a^2+b^2.$
Id est, $$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geqfrac{1}{2}(a^2+b^2)left(frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}right).$$
Now, let $a=tb$.
Thus, we need to prove that
$$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ or
$$(t-1)^2(16t^4+16t^3-7t^2-2t+1)geq0,$$ which is obviously true for $tgeq1.$
But for $0<tleq1$ we obtain:
$$16t^4+16t^3-7t^2-2t+1=$$
$$=9t^3(1-t)+left(5t^2+frac{7}{10}t-frac{5}{6}right)^2+frac{1}{900}(759t^2-750t+275)geq0.$$
Done!
Also, we can use derivative here.
Since $$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ it's $f(t)geq0,$ where
$$f(t)=64t^6-64t^5-92t^4+112t^3-8t^2-16t+4,$$ and
$$f'(t)=(t-1)(t+1)(3t-1)(8t^2-4t-1),$$
we obtain $t_{min}=frac{1}{3}$ or $t_{min}=1$ and since $fleft(frac{1}{3}right)>f(1)=0$, we are done.
$endgroup$
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
add a comment |
$begingroup$
Let $x=2$ and $y=1$.
Thus, we'll get a value $6$.
We'll prove that it's a minimal value.
Indeed, let $x=2a$ and $y=b$.
Thus, $3a^2+2b^3=5$ and we need to prove that
$$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geq6.$$
But by AM-GM
$$5=3a^2+2b^3=3a^2-1+3b^2+2b^3+1-3b^2geq$$
$$geq3a^2+3b^2-1+3sqrt[3]{(b^3)^2cdot1}-3b^2=3(a^2+b^2)-1,$$
which gives $2geq a^2+b^2.$
Id est, $$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geqfrac{1}{2}(a^2+b^2)left(frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}right).$$
Now, let $a=tb$.
Thus, we need to prove that
$$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ or
$$(t-1)^2(16t^4+16t^3-7t^2-2t+1)geq0,$$ which is obviously true for $tgeq1.$
But for $0<tleq1$ we obtain:
$$16t^4+16t^3-7t^2-2t+1=$$
$$=9t^3(1-t)+left(5t^2+frac{7}{10}t-frac{5}{6}right)^2+frac{1}{900}(759t^2-750t+275)geq0.$$
Done!
Also, we can use derivative here.
Since $$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ it's $f(t)geq0,$ where
$$f(t)=64t^6-64t^5-92t^4+112t^3-8t^2-16t+4,$$ and
$$f'(t)=(t-1)(t+1)(3t-1)(8t^2-4t-1),$$
we obtain $t_{min}=frac{1}{3}$ or $t_{min}=1$ and since $fleft(frac{1}{3}right)>f(1)=0$, we are done.
$endgroup$
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
add a comment |
$begingroup$
Let $x=2$ and $y=1$.
Thus, we'll get a value $6$.
We'll prove that it's a minimal value.
Indeed, let $x=2a$ and $y=b$.
Thus, $3a^2+2b^3=5$ and we need to prove that
$$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geq6.$$
But by AM-GM
$$5=3a^2+2b^3=3a^2-1+3b^2+2b^3+1-3b^2geq$$
$$geq3a^2+3b^2-1+3sqrt[3]{(b^3)^2cdot1}-3b^2=3(a^2+b^2)-1,$$
which gives $2geq a^2+b^2.$
Id est, $$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geqfrac{1}{2}(a^2+b^2)left(frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}right).$$
Now, let $a=tb$.
Thus, we need to prove that
$$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ or
$$(t-1)^2(16t^4+16t^3-7t^2-2t+1)geq0,$$ which is obviously true for $tgeq1.$
But for $0<tleq1$ we obtain:
$$16t^4+16t^3-7t^2-2t+1=$$
$$=9t^3(1-t)+left(5t^2+frac{7}{10}t-frac{5}{6}right)^2+frac{1}{900}(759t^2-750t+275)geq0.$$
Done!
Also, we can use derivative here.
Since $$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ it's $f(t)geq0,$ where
$$f(t)=64t^6-64t^5-92t^4+112t^3-8t^2-16t+4,$$ and
$$f'(t)=(t-1)(t+1)(3t-1)(8t^2-4t-1),$$
we obtain $t_{min}=frac{1}{3}$ or $t_{min}=1$ and since $fleft(frac{1}{3}right)>f(1)=0$, we are done.
$endgroup$
Let $x=2$ and $y=1$.
Thus, we'll get a value $6$.
We'll prove that it's a minimal value.
Indeed, let $x=2a$ and $y=b$.
Thus, $3a^2+2b^3=5$ and we need to prove that
$$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geq6.$$
But by AM-GM
$$5=3a^2+2b^3=3a^2-1+3b^2+2b^3+1-3b^2geq$$
$$geq3a^2+3b^2-1+3sqrt[3]{(b^3)^2cdot1}-3b^2=3(a^2+b^2)-1,$$
which gives $2geq a^2+b^2.$
Id est, $$frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}geqfrac{1}{2}(a^2+b^2)left(frac{1}{a^2}+frac{4}{b^2}+frac{1}{(2a-b)^2}right).$$
Now, let $a=tb$.
Thus, we need to prove that
$$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ or
$$(t-1)^2(16t^4+16t^3-7t^2-2t+1)geq0,$$ which is obviously true for $tgeq1.$
But for $0<tleq1$ we obtain:
$$16t^4+16t^3-7t^2-2t+1=$$
$$=9t^3(1-t)+left(5t^2+frac{7}{10}t-frac{5}{6}right)^2+frac{1}{900}(759t^2-750t+275)geq0.$$
Done!
Also, we can use derivative here.
Since $$(t^2+1)left(frac{1}{t^2}+4+frac{1}{(2t-1)^2}right)geq12$$ it's $f(t)geq0,$ where
$$f(t)=64t^6-64t^5-92t^4+112t^3-8t^2-16t+4,$$ and
$$f'(t)=(t-1)(t+1)(3t-1)(8t^2-4t-1),$$
we obtain $t_{min}=frac{1}{3}$ or $t_{min}=1$ and since $fleft(frac{1}{3}right)>f(1)=0$, we are done.
edited Dec 13 '18 at 13:49
answered Dec 13 '18 at 12:04
Michael RozenbergMichael Rozenberg
106k1893198
106k1893198
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
add a comment |
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
Can w use derivative to solve this. Thank you
$endgroup$
– Trong Tuan
Dec 13 '18 at 13:28
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
@Trong Tuan I added something. See now.
$endgroup$
– Michael Rozenberg
Dec 13 '18 at 13:50
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
$begingroup$
In $$x=2,; y=1$$ is local minimum. Global minimum is in $x=-1.668453756732446,;y=1.133435720989714$
$endgroup$
– Aleksas Domarkas
Dec 19 '18 at 11:38
add a comment |
$begingroup$
Local minimum:
$$f(2,1)=6$$
Global minimum:
$$f(−1.668453756732446,1.133435720989714)=4.677920159716602$$
$endgroup$
add a comment |
$begingroup$
Local minimum:
$$f(2,1)=6$$
Global minimum:
$$f(−1.668453756732446,1.133435720989714)=4.677920159716602$$
$endgroup$
add a comment |
$begingroup$
Local minimum:
$$f(2,1)=6$$
Global minimum:
$$f(−1.668453756732446,1.133435720989714)=4.677920159716602$$
$endgroup$
Local minimum:
$$f(2,1)=6$$
Global minimum:
$$f(−1.668453756732446,1.133435720989714)=4.677920159716602$$
answered Dec 19 '18 at 20:43
Aleksas DomarkasAleksas Domarkas
1,37216
1,37216
add a comment |
add a comment |
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What does > mean in the second line?
$endgroup$
– William Elliot
Dec 13 '18 at 11:55