finding minimum value of the expression
What is the minimum value of the expression
$$(a+1/a)^2+(b+1/b)^2+(c+1/c)^2$$
How do we go ahead solving this problem ? Can we use $AMge HM$ to solve this problem ?The answer is supposed to be 12 which i can see in answer booklet but i have no clue how to solve this even when i know the concept of AM>=HM.
inequality optimization arithmetic
add a comment |
What is the minimum value of the expression
$$(a+1/a)^2+(b+1/b)^2+(c+1/c)^2$$
How do we go ahead solving this problem ? Can we use $AMge HM$ to solve this problem ?The answer is supposed to be 12 which i can see in answer booklet but i have no clue how to solve this even when i know the concept of AM>=HM.
inequality optimization arithmetic
And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago
add a comment |
What is the minimum value of the expression
$$(a+1/a)^2+(b+1/b)^2+(c+1/c)^2$$
How do we go ahead solving this problem ? Can we use $AMge HM$ to solve this problem ?The answer is supposed to be 12 which i can see in answer booklet but i have no clue how to solve this even when i know the concept of AM>=HM.
inequality optimization arithmetic
What is the minimum value of the expression
$$(a+1/a)^2+(b+1/b)^2+(c+1/c)^2$$
How do we go ahead solving this problem ? Can we use $AMge HM$ to solve this problem ?The answer is supposed to be 12 which i can see in answer booklet but i have no clue how to solve this even when i know the concept of AM>=HM.
inequality optimization arithmetic
inequality optimization arithmetic
edited 6 hours ago
greedoid
37.9k114794
37.9k114794
asked 7 hours ago
iamredpanda
123
123
And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago
add a comment |
And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago
And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago
add a comment |
3 Answers
3
active
oldest
votes
Since $$(x+1/x)^2 = underbrace{x^2+1/x^2}_{geq 2}+2geq 2+2=4$$ your expression is at least 12
and equality is achieved if $a=b=c=1$.
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
add a comment |
Opimization is another way to solve this.
$$f(a) = a+frac{1}{a} implies f’(a) = 1-a^{-2}$$
Setting $f’(a) = 0$ to obtain the minimum, you have
$$0 = 1-a^{-2} implies a^{-2} = 1 implies a = pm 1$$
(For clarification, $a = -1$ gives the minimum absolute value for negative values of $a$.)
Thus, the minimum is obtained when $a = pm 1$, so you have $left(a+frac{1}{a}right)^2 = (pm 2)^2 = 4$. The same goes for $b$ and $c$, so the minimum occurs when $a = b = c = pm 1$ and is $4+4+4 = 12$.
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
add a comment |
The given expression equals
begin{align*}
a^2 + b^2 + c^2 &+ frac{1}{a^2} + frac{1}{b^2} + frac{1}{c^2} + 6 \
&geq 6 left(a^2 cdot b^2 cdot c^2 cdot frac{1}{a^2}frac{1}{b^2}frac{1}{c^2} right)^{frac{1}{6}} + 6 \
&= 12
end{align*}
where we have used $AM - GM$ inequality.
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
Since $$(x+1/x)^2 = underbrace{x^2+1/x^2}_{geq 2}+2geq 2+2=4$$ your expression is at least 12
and equality is achieved if $a=b=c=1$.
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
add a comment |
Since $$(x+1/x)^2 = underbrace{x^2+1/x^2}_{geq 2}+2geq 2+2=4$$ your expression is at least 12
and equality is achieved if $a=b=c=1$.
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
add a comment |
Since $$(x+1/x)^2 = underbrace{x^2+1/x^2}_{geq 2}+2geq 2+2=4$$ your expression is at least 12
and equality is achieved if $a=b=c=1$.
Since $$(x+1/x)^2 = underbrace{x^2+1/x^2}_{geq 2}+2geq 2+2=4$$ your expression is at least 12
and equality is achieved if $a=b=c=1$.
answered 6 hours ago
greedoid
37.9k114794
37.9k114794
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
add a comment |
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
Oh this turned out to be too simple.I was trying to think in wrong direction .
– iamredpanda
6 hours ago
1
1
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
How do you show $x^2+1/x^2ge2$? Won't you need the same steps that can also show $x+1/xge2$?
– Teepeemm
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
Exactly the same, just write $a=x^2$.
– greedoid
5 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
@Teepeemm , with squares we are sure the numbers are positive. It is not so for $x+1/x$ which is less than $-2$ if $x<0.$
– user376343
4 hours ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
Then what does your answer add? If the same steps can show $x+1/xge2$, then how does this answer help OP? @user376343 Squares being positive shows us that $x^2+1/x^2ge0$, not $ge2$.
– Teepeemm
20 mins ago
add a comment |
Opimization is another way to solve this.
$$f(a) = a+frac{1}{a} implies f’(a) = 1-a^{-2}$$
Setting $f’(a) = 0$ to obtain the minimum, you have
$$0 = 1-a^{-2} implies a^{-2} = 1 implies a = pm 1$$
(For clarification, $a = -1$ gives the minimum absolute value for negative values of $a$.)
Thus, the minimum is obtained when $a = pm 1$, so you have $left(a+frac{1}{a}right)^2 = (pm 2)^2 = 4$. The same goes for $b$ and $c$, so the minimum occurs when $a = b = c = pm 1$ and is $4+4+4 = 12$.
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
add a comment |
Opimization is another way to solve this.
$$f(a) = a+frac{1}{a} implies f’(a) = 1-a^{-2}$$
Setting $f’(a) = 0$ to obtain the minimum, you have
$$0 = 1-a^{-2} implies a^{-2} = 1 implies a = pm 1$$
(For clarification, $a = -1$ gives the minimum absolute value for negative values of $a$.)
Thus, the minimum is obtained when $a = pm 1$, so you have $left(a+frac{1}{a}right)^2 = (pm 2)^2 = 4$. The same goes for $b$ and $c$, so the minimum occurs when $a = b = c = pm 1$ and is $4+4+4 = 12$.
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
add a comment |
Opimization is another way to solve this.
$$f(a) = a+frac{1}{a} implies f’(a) = 1-a^{-2}$$
Setting $f’(a) = 0$ to obtain the minimum, you have
$$0 = 1-a^{-2} implies a^{-2} = 1 implies a = pm 1$$
(For clarification, $a = -1$ gives the minimum absolute value for negative values of $a$.)
Thus, the minimum is obtained when $a = pm 1$, so you have $left(a+frac{1}{a}right)^2 = (pm 2)^2 = 4$. The same goes for $b$ and $c$, so the minimum occurs when $a = b = c = pm 1$ and is $4+4+4 = 12$.
Opimization is another way to solve this.
$$f(a) = a+frac{1}{a} implies f’(a) = 1-a^{-2}$$
Setting $f’(a) = 0$ to obtain the minimum, you have
$$0 = 1-a^{-2} implies a^{-2} = 1 implies a = pm 1$$
(For clarification, $a = -1$ gives the minimum absolute value for negative values of $a$.)
Thus, the minimum is obtained when $a = pm 1$, so you have $left(a+frac{1}{a}right)^2 = (pm 2)^2 = 4$. The same goes for $b$ and $c$, so the minimum occurs when $a = b = c = pm 1$ and is $4+4+4 = 12$.
edited 6 hours ago
answered 6 hours ago
KM101
4,886421
4,886421
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
add a comment |
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
why does $f'(a)=0$ coincide with a minimum?
– LinAlg
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
Because the stationary points occur where $f’(a) = 0$.
– KM101
6 hours ago
2
2
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
stationary points are not necessarily minima, and the function may not even be bounded below
– LinAlg
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
I guess my reply wasn't clear enough. I didn't mean a stationary point always implies minima, but that it is the case in this example. For instance, for positive $a$, there is $frac{bpm n}{b}+frac{b}{bpm n} = frac{2left(b^2-bnright)+n^2}{b^2-bn}$, so there clearly is a minimum where $n = 0$, so $a = 1$.
– KM101
5 hours ago
add a comment |
The given expression equals
begin{align*}
a^2 + b^2 + c^2 &+ frac{1}{a^2} + frac{1}{b^2} + frac{1}{c^2} + 6 \
&geq 6 left(a^2 cdot b^2 cdot c^2 cdot frac{1}{a^2}frac{1}{b^2}frac{1}{c^2} right)^{frac{1}{6}} + 6 \
&= 12
end{align*}
where we have used $AM - GM$ inequality.
add a comment |
The given expression equals
begin{align*}
a^2 + b^2 + c^2 &+ frac{1}{a^2} + frac{1}{b^2} + frac{1}{c^2} + 6 \
&geq 6 left(a^2 cdot b^2 cdot c^2 cdot frac{1}{a^2}frac{1}{b^2}frac{1}{c^2} right)^{frac{1}{6}} + 6 \
&= 12
end{align*}
where we have used $AM - GM$ inequality.
add a comment |
The given expression equals
begin{align*}
a^2 + b^2 + c^2 &+ frac{1}{a^2} + frac{1}{b^2} + frac{1}{c^2} + 6 \
&geq 6 left(a^2 cdot b^2 cdot c^2 cdot frac{1}{a^2}frac{1}{b^2}frac{1}{c^2} right)^{frac{1}{6}} + 6 \
&= 12
end{align*}
where we have used $AM - GM$ inequality.
The given expression equals
begin{align*}
a^2 + b^2 + c^2 &+ frac{1}{a^2} + frac{1}{b^2} + frac{1}{c^2} + 6 \
&geq 6 left(a^2 cdot b^2 cdot c^2 cdot frac{1}{a^2}frac{1}{b^2}frac{1}{c^2} right)^{frac{1}{6}} + 6 \
&= 12
end{align*}
where we have used $AM - GM$ inequality.
answered 6 hours ago
Muralidharan
48516
48516
add a comment |
add a comment |
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And $$a,b,c$$ assumed to be positive?
– Dr. Sonnhard Graubner
7 hours ago
$a = b = c = -1$ or $1$.
– David G. Stork
6 hours ago
I think it must be additionally $$a+b+c=1$$ and $$a>0,b>0,c>0$$
– Dr. Sonnhard Graubner
6 hours ago
See also: math.stackexchange.com/questions/487486/…
– lab bhattacharjee
4 hours ago