A question on the parabola
$f:[-2,5] to mathbb{R}$
$f(x)= x^2-2mx+m+2$ The smallest value that the function can take is $-10$. Find the sum of the values that $m$ can take.
algebra-precalculus
add a comment |
$f:[-2,5] to mathbb{R}$
$f(x)= x^2-2mx+m+2$ The smallest value that the function can take is $-10$. Find the sum of the values that $m$ can take.
algebra-precalculus
The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45
add a comment |
$f:[-2,5] to mathbb{R}$
$f(x)= x^2-2mx+m+2$ The smallest value that the function can take is $-10$. Find the sum of the values that $m$ can take.
algebra-precalculus
$f:[-2,5] to mathbb{R}$
$f(x)= x^2-2mx+m+2$ The smallest value that the function can take is $-10$. Find the sum of the values that $m$ can take.
algebra-precalculus
algebra-precalculus
edited Dec 9 at 15:45
mm-crj
382212
382212
asked Dec 9 at 15:25
Hatm00
276
276
The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45
add a comment |
The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45
The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45
add a comment |
2 Answers
2
active
oldest
votes
The smallest value of $ax^2+bx+c$ (when $a>0$) is taken at $x$ halfway between the roots, which is
$$
x=-frac{b}{2a}=m
$$
(For $a<0$, that abscissa gives the maximum value.)
However, your problem is a bit more complicated, because you bound the function over $[-2,5]$. Then the minimum value can be at $m$ (if it is in this range), at $-2$ or at $5$.
Now you have $f(-2)=5m+6$ and $f(5)=-9m+27$. Also
$$
f(m)=m^2-2m^2+m+2=-m^2+m+2
$$
- If $-2<m<5$, the minimum value is taken at $m$
- if $mle-2$, the minimum is taken at …
- if $mge5$, the minimum is taken at …
We have $f(m)=-10$ if and only if $m^2-m-12=0$, that is $m=4$ or $m=-3$.
We have $f(-2)=-10$ if and only if $m=-16/5<-2$.
We have $f(5)=-10$ if and only if $m=37/9$.
Two of these values have to be discarded.
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
|
show 1 more comment
The maximum or minimum of a parabola occurs at its vertex at $(h, k)$.
$$h = frac{-b}{2a}$$
$$k = c-frac{b^2}{4a}$$
Since you have $a = 1 > 0$, then a minimum exists.
$$k = -10 iff c-frac{b^2}{4a} = -10 iff (m+2)-frac{4m^2}{4} = -10$$
$$frac{4m+8-4m^2}{4} = -10 iff -4m^2+4m+8 = -40$$
$$-4m^2+4m+48 = 0 iff m^2-m-12 = 0 implies (m-4)(m+3) = 0$$
This yields $m = 4$ and $m = -3$, but only the former is applicable given the limited domain.
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
oldest
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active
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votes
The smallest value of $ax^2+bx+c$ (when $a>0$) is taken at $x$ halfway between the roots, which is
$$
x=-frac{b}{2a}=m
$$
(For $a<0$, that abscissa gives the maximum value.)
However, your problem is a bit more complicated, because you bound the function over $[-2,5]$. Then the minimum value can be at $m$ (if it is in this range), at $-2$ or at $5$.
Now you have $f(-2)=5m+6$ and $f(5)=-9m+27$. Also
$$
f(m)=m^2-2m^2+m+2=-m^2+m+2
$$
- If $-2<m<5$, the minimum value is taken at $m$
- if $mle-2$, the minimum is taken at …
- if $mge5$, the minimum is taken at …
We have $f(m)=-10$ if and only if $m^2-m-12=0$, that is $m=4$ or $m=-3$.
We have $f(-2)=-10$ if and only if $m=-16/5<-2$.
We have $f(5)=-10$ if and only if $m=37/9$.
Two of these values have to be discarded.
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
|
show 1 more comment
The smallest value of $ax^2+bx+c$ (when $a>0$) is taken at $x$ halfway between the roots, which is
$$
x=-frac{b}{2a}=m
$$
(For $a<0$, that abscissa gives the maximum value.)
However, your problem is a bit more complicated, because you bound the function over $[-2,5]$. Then the minimum value can be at $m$ (if it is in this range), at $-2$ or at $5$.
Now you have $f(-2)=5m+6$ and $f(5)=-9m+27$. Also
$$
f(m)=m^2-2m^2+m+2=-m^2+m+2
$$
- If $-2<m<5$, the minimum value is taken at $m$
- if $mle-2$, the minimum is taken at …
- if $mge5$, the minimum is taken at …
We have $f(m)=-10$ if and only if $m^2-m-12=0$, that is $m=4$ or $m=-3$.
We have $f(-2)=-10$ if and only if $m=-16/5<-2$.
We have $f(5)=-10$ if and only if $m=37/9$.
Two of these values have to be discarded.
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
|
show 1 more comment
The smallest value of $ax^2+bx+c$ (when $a>0$) is taken at $x$ halfway between the roots, which is
$$
x=-frac{b}{2a}=m
$$
(For $a<0$, that abscissa gives the maximum value.)
However, your problem is a bit more complicated, because you bound the function over $[-2,5]$. Then the minimum value can be at $m$ (if it is in this range), at $-2$ or at $5$.
Now you have $f(-2)=5m+6$ and $f(5)=-9m+27$. Also
$$
f(m)=m^2-2m^2+m+2=-m^2+m+2
$$
- If $-2<m<5$, the minimum value is taken at $m$
- if $mle-2$, the minimum is taken at …
- if $mge5$, the minimum is taken at …
We have $f(m)=-10$ if and only if $m^2-m-12=0$, that is $m=4$ or $m=-3$.
We have $f(-2)=-10$ if and only if $m=-16/5<-2$.
We have $f(5)=-10$ if and only if $m=37/9$.
Two of these values have to be discarded.
The smallest value of $ax^2+bx+c$ (when $a>0$) is taken at $x$ halfway between the roots, which is
$$
x=-frac{b}{2a}=m
$$
(For $a<0$, that abscissa gives the maximum value.)
However, your problem is a bit more complicated, because you bound the function over $[-2,5]$. Then the minimum value can be at $m$ (if it is in this range), at $-2$ or at $5$.
Now you have $f(-2)=5m+6$ and $f(5)=-9m+27$. Also
$$
f(m)=m^2-2m^2+m+2=-m^2+m+2
$$
- If $-2<m<5$, the minimum value is taken at $m$
- if $mle-2$, the minimum is taken at …
- if $mge5$, the minimum is taken at …
We have $f(m)=-10$ if and only if $m^2-m-12=0$, that is $m=4$ or $m=-3$.
We have $f(-2)=-10$ if and only if $m=-16/5<-2$.
We have $f(5)=-10$ if and only if $m=37/9$.
Two of these values have to be discarded.
edited Dec 9 at 16:19
answered Dec 9 at 15:28
egreg
178k1484201
178k1484201
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
|
show 1 more comment
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
When the root exists, if the first claim aims to be general (since we are in a precalculus context). Or are you assuming that we can refer also to the compelx roots in that context?
– gimusi
Dec 9 at 15:32
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
Hey smallest value can also occur at boundary right ?
– rsadhvika
Dec 9 at 15:34
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
But it's a delimited function. It can be equal f(-2) or f(5). Is my thought wrong?
– Hatm00
Dec 9 at 15:42
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@gimusi Shifting the parabola up or down doesn't change the point where the minimum is taken, only the minimum value. The halfway point doesn't change with such a translation.
– egreg
Dec 9 at 16:04
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
@Hatm00 I added further hints
– egreg
Dec 9 at 16:19
|
show 1 more comment
The maximum or minimum of a parabola occurs at its vertex at $(h, k)$.
$$h = frac{-b}{2a}$$
$$k = c-frac{b^2}{4a}$$
Since you have $a = 1 > 0$, then a minimum exists.
$$k = -10 iff c-frac{b^2}{4a} = -10 iff (m+2)-frac{4m^2}{4} = -10$$
$$frac{4m+8-4m^2}{4} = -10 iff -4m^2+4m+8 = -40$$
$$-4m^2+4m+48 = 0 iff m^2-m-12 = 0 implies (m-4)(m+3) = 0$$
This yields $m = 4$ and $m = -3$, but only the former is applicable given the limited domain.
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
add a comment |
The maximum or minimum of a parabola occurs at its vertex at $(h, k)$.
$$h = frac{-b}{2a}$$
$$k = c-frac{b^2}{4a}$$
Since you have $a = 1 > 0$, then a minimum exists.
$$k = -10 iff c-frac{b^2}{4a} = -10 iff (m+2)-frac{4m^2}{4} = -10$$
$$frac{4m+8-4m^2}{4} = -10 iff -4m^2+4m+8 = -40$$
$$-4m^2+4m+48 = 0 iff m^2-m-12 = 0 implies (m-4)(m+3) = 0$$
This yields $m = 4$ and $m = -3$, but only the former is applicable given the limited domain.
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
add a comment |
The maximum or minimum of a parabola occurs at its vertex at $(h, k)$.
$$h = frac{-b}{2a}$$
$$k = c-frac{b^2}{4a}$$
Since you have $a = 1 > 0$, then a minimum exists.
$$k = -10 iff c-frac{b^2}{4a} = -10 iff (m+2)-frac{4m^2}{4} = -10$$
$$frac{4m+8-4m^2}{4} = -10 iff -4m^2+4m+8 = -40$$
$$-4m^2+4m+48 = 0 iff m^2-m-12 = 0 implies (m-4)(m+3) = 0$$
This yields $m = 4$ and $m = -3$, but only the former is applicable given the limited domain.
The maximum or minimum of a parabola occurs at its vertex at $(h, k)$.
$$h = frac{-b}{2a}$$
$$k = c-frac{b^2}{4a}$$
Since you have $a = 1 > 0$, then a minimum exists.
$$k = -10 iff c-frac{b^2}{4a} = -10 iff (m+2)-frac{4m^2}{4} = -10$$
$$frac{4m+8-4m^2}{4} = -10 iff -4m^2+4m+8 = -40$$
$$-4m^2+4m+48 = 0 iff m^2-m-12 = 0 implies (m-4)(m+3) = 0$$
This yields $m = 4$ and $m = -3$, but only the former is applicable given the limited domain.
edited Dec 9 at 15:47
answered Dec 9 at 15:35
KM101
4,836421
4,836421
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
add a comment |
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
But we don't know is vertex in definition range.
– Hatm00
Dec 9 at 15:39
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
@Hatm00 I’ve edited the answer. (I misread the question.)
– KM101
Dec 9 at 15:51
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
Thanks so much for your help. If we add root with the boundary values we can reach the result.
– Hatm00
Dec 9 at 16:01
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
It would be good if the downvoter clarifies if the answer is missing anything.
– KM101
Dec 9 at 16:03
add a comment |
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The smallest value occurs at the vertex of the parabola. Do you know how to find the vertex?
– rogerl
Dec 9 at 15:27
You do know you can write text outside the dollar signs, right? You don't have to have a backslash before every space, and it looks better.
– Arthur
Dec 9 at 15:28
@rogerl I tried but it didn't work out.
– Hatm00
Dec 9 at 15:33
@Hatm00 keep in mind the minimum value can also occur at boundary, see
– rsadhvika
Dec 9 at 15:42
You're right but also I found the values that you found and vertex value. As I said it didn't give the result.
– Hatm00
Dec 9 at 15:45