$f(2x)=f(x+y)f(x-y)$ for all $x,y in mathbb{R}$
$begingroup$
$f(2x)=f(x+y)f(x-y)$ for all $x,y in mathbb{R}$, and we also know $f(10)=4, f(0)neq 0$, and $f'(0)=2$.
Then what is $f'(10)$ ...?
functions
$endgroup$
add a comment |
$begingroup$
$f(2x)=f(x+y)f(x-y)$ for all $x,y in mathbb{R}$, and we also know $f(10)=4, f(0)neq 0$, and $f'(0)=2$.
Then what is $f'(10)$ ...?
functions
$endgroup$
3
$begingroup$
Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22
add a comment |
$begingroup$
$f(2x)=f(x+y)f(x-y)$ for all $x,y in mathbb{R}$, and we also know $f(10)=4, f(0)neq 0$, and $f'(0)=2$.
Then what is $f'(10)$ ...?
functions
$endgroup$
$f(2x)=f(x+y)f(x-y)$ for all $x,y in mathbb{R}$, and we also know $f(10)=4, f(0)neq 0$, and $f'(0)=2$.
Then what is $f'(10)$ ...?
functions
functions
edited Dec 7 '18 at 7:33
InsideOut
4,96631033
4,96631033
asked Dec 7 '18 at 7:16
Apple PieApple Pie
32
32
3
$begingroup$
Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22
add a comment |
3
$begingroup$
Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22
3
3
$begingroup$
Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22
$begingroup$
Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Differntiate w.r.t. $y$ to get $f(x+y)f'(x-y)=f'(x+y)f(x-y)$. Put $x=y$ to get $2f(2x)=f'(2x)f(0)$. Hence $2f(x)=f'(x)f(0)$ for all $x$. Solving this we get $f(x) =ae^{cx}$. Can you take it from here?.
$endgroup$
add a comment |
$begingroup$
Let $x=y=0$, then
$$f(0)=f(0)f(0)$$
which implies $f(0)=1$ when $f(0)neq 0$.
If $f(x)$ is differentiable then we have
$$2f'(2x)=f'(x+y)f(x-y)+f(x+y)f'(x-y)$$
let $x=y=5$, then
$$2f'(10)=f'(10)f(0)+f(10)f'(0)=f'(10)+8$$
hence we have $f'(10)=8$.
$endgroup$
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
add a comment |
$begingroup$
We need not assume that $f$ is differentiable or even continuous except at $0$.
With $x=frac{u+v}2$, $y=frac{u-v}2$, we obtain
$$tag1 f(u+v)=f(u)f(v)qquad text{for all }u,vinBbb R .$$
Then also
$$tag2operatorname{sgn}(f(u+v))= operatorname{sgn}(f(u))operatorname{sgn}(f(v))qquad text{for all }u,vinBbb R .$$
As $f$ is continuous at $0$ and $f(0)ne 0$, $operatorname{sgn}circ f$ is constant in a neighbourhood of $0$. Then from $(2)$ with $vapprox 0$, we see that $operatorname{sgn}circ f$ is locally constant, hence constant throughout, and finally must be $=1$, i.e.,
$$tag3f(x)>0qquad text{for all }xinBbb R.$$
Hence we can define $g(x)=ln f(x)$ for all $xin Bbb R$ and conclude from $(1)$ that $g$ is additive.
It is well-known that continuity at a single point (as here implied by the existence of $f'(0)$) implies that all solutions are of the form
$$ g(x)=cx.$$
Therefore
$$ f(x)=e^{cx}$$
for a suitable constant $c$.
It follows that $f$ is smooth and that
$$ f'(x)=cf(x)$$
so that
$$f'(10)=frac{f'(0)}{f(0)}f(10)=8. $$
$endgroup$
add a comment |
Your Answer
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3 Answers
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active
oldest
votes
3 Answers
3
active
oldest
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votes
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votes
$begingroup$
Differntiate w.r.t. $y$ to get $f(x+y)f'(x-y)=f'(x+y)f(x-y)$. Put $x=y$ to get $2f(2x)=f'(2x)f(0)$. Hence $2f(x)=f'(x)f(0)$ for all $x$. Solving this we get $f(x) =ae^{cx}$. Can you take it from here?.
$endgroup$
add a comment |
$begingroup$
Differntiate w.r.t. $y$ to get $f(x+y)f'(x-y)=f'(x+y)f(x-y)$. Put $x=y$ to get $2f(2x)=f'(2x)f(0)$. Hence $2f(x)=f'(x)f(0)$ for all $x$. Solving this we get $f(x) =ae^{cx}$. Can you take it from here?.
$endgroup$
add a comment |
$begingroup$
Differntiate w.r.t. $y$ to get $f(x+y)f'(x-y)=f'(x+y)f(x-y)$. Put $x=y$ to get $2f(2x)=f'(2x)f(0)$. Hence $2f(x)=f'(x)f(0)$ for all $x$. Solving this we get $f(x) =ae^{cx}$. Can you take it from here?.
$endgroup$
Differntiate w.r.t. $y$ to get $f(x+y)f'(x-y)=f'(x+y)f(x-y)$. Put $x=y$ to get $2f(2x)=f'(2x)f(0)$. Hence $2f(x)=f'(x)f(0)$ for all $x$. Solving this we get $f(x) =ae^{cx}$. Can you take it from here?.
answered Dec 7 '18 at 7:26
Kavi Rama MurthyKavi Rama Murthy
58.2k42160
58.2k42160
add a comment |
add a comment |
$begingroup$
Let $x=y=0$, then
$$f(0)=f(0)f(0)$$
which implies $f(0)=1$ when $f(0)neq 0$.
If $f(x)$ is differentiable then we have
$$2f'(2x)=f'(x+y)f(x-y)+f(x+y)f'(x-y)$$
let $x=y=5$, then
$$2f'(10)=f'(10)f(0)+f(10)f'(0)=f'(10)+8$$
hence we have $f'(10)=8$.
$endgroup$
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
add a comment |
$begingroup$
Let $x=y=0$, then
$$f(0)=f(0)f(0)$$
which implies $f(0)=1$ when $f(0)neq 0$.
If $f(x)$ is differentiable then we have
$$2f'(2x)=f'(x+y)f(x-y)+f(x+y)f'(x-y)$$
let $x=y=5$, then
$$2f'(10)=f'(10)f(0)+f(10)f'(0)=f'(10)+8$$
hence we have $f'(10)=8$.
$endgroup$
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
add a comment |
$begingroup$
Let $x=y=0$, then
$$f(0)=f(0)f(0)$$
which implies $f(0)=1$ when $f(0)neq 0$.
If $f(x)$ is differentiable then we have
$$2f'(2x)=f'(x+y)f(x-y)+f(x+y)f'(x-y)$$
let $x=y=5$, then
$$2f'(10)=f'(10)f(0)+f(10)f'(0)=f'(10)+8$$
hence we have $f'(10)=8$.
$endgroup$
Let $x=y=0$, then
$$f(0)=f(0)f(0)$$
which implies $f(0)=1$ when $f(0)neq 0$.
If $f(x)$ is differentiable then we have
$$2f'(2x)=f'(x+y)f(x-y)+f(x+y)f'(x-y)$$
let $x=y=5$, then
$$2f'(10)=f'(10)f(0)+f(10)f'(0)=f'(10)+8$$
hence we have $f'(10)=8$.
answered Dec 7 '18 at 7:26
LauLau
527315
527315
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
add a comment |
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
Can u tell me what are u differentiating the equation with respect to ?
$endgroup$
– Apple Pie
Dec 8 '18 at 14:09
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
with respect to $x$
$endgroup$
– Lau
Dec 9 '18 at 2:05
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
Sry but im just confused why isn’t there a y’. Is it considered constant
$endgroup$
– Apple Pie
Dec 9 '18 at 2:58
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
$begingroup$
I think $x,y$ are independent.
$endgroup$
– Lau
Dec 9 '18 at 6:35
add a comment |
$begingroup$
We need not assume that $f$ is differentiable or even continuous except at $0$.
With $x=frac{u+v}2$, $y=frac{u-v}2$, we obtain
$$tag1 f(u+v)=f(u)f(v)qquad text{for all }u,vinBbb R .$$
Then also
$$tag2operatorname{sgn}(f(u+v))= operatorname{sgn}(f(u))operatorname{sgn}(f(v))qquad text{for all }u,vinBbb R .$$
As $f$ is continuous at $0$ and $f(0)ne 0$, $operatorname{sgn}circ f$ is constant in a neighbourhood of $0$. Then from $(2)$ with $vapprox 0$, we see that $operatorname{sgn}circ f$ is locally constant, hence constant throughout, and finally must be $=1$, i.e.,
$$tag3f(x)>0qquad text{for all }xinBbb R.$$
Hence we can define $g(x)=ln f(x)$ for all $xin Bbb R$ and conclude from $(1)$ that $g$ is additive.
It is well-known that continuity at a single point (as here implied by the existence of $f'(0)$) implies that all solutions are of the form
$$ g(x)=cx.$$
Therefore
$$ f(x)=e^{cx}$$
for a suitable constant $c$.
It follows that $f$ is smooth and that
$$ f'(x)=cf(x)$$
so that
$$f'(10)=frac{f'(0)}{f(0)}f(10)=8. $$
$endgroup$
add a comment |
$begingroup$
We need not assume that $f$ is differentiable or even continuous except at $0$.
With $x=frac{u+v}2$, $y=frac{u-v}2$, we obtain
$$tag1 f(u+v)=f(u)f(v)qquad text{for all }u,vinBbb R .$$
Then also
$$tag2operatorname{sgn}(f(u+v))= operatorname{sgn}(f(u))operatorname{sgn}(f(v))qquad text{for all }u,vinBbb R .$$
As $f$ is continuous at $0$ and $f(0)ne 0$, $operatorname{sgn}circ f$ is constant in a neighbourhood of $0$. Then from $(2)$ with $vapprox 0$, we see that $operatorname{sgn}circ f$ is locally constant, hence constant throughout, and finally must be $=1$, i.e.,
$$tag3f(x)>0qquad text{for all }xinBbb R.$$
Hence we can define $g(x)=ln f(x)$ for all $xin Bbb R$ and conclude from $(1)$ that $g$ is additive.
It is well-known that continuity at a single point (as here implied by the existence of $f'(0)$) implies that all solutions are of the form
$$ g(x)=cx.$$
Therefore
$$ f(x)=e^{cx}$$
for a suitable constant $c$.
It follows that $f$ is smooth and that
$$ f'(x)=cf(x)$$
so that
$$f'(10)=frac{f'(0)}{f(0)}f(10)=8. $$
$endgroup$
add a comment |
$begingroup$
We need not assume that $f$ is differentiable or even continuous except at $0$.
With $x=frac{u+v}2$, $y=frac{u-v}2$, we obtain
$$tag1 f(u+v)=f(u)f(v)qquad text{for all }u,vinBbb R .$$
Then also
$$tag2operatorname{sgn}(f(u+v))= operatorname{sgn}(f(u))operatorname{sgn}(f(v))qquad text{for all }u,vinBbb R .$$
As $f$ is continuous at $0$ and $f(0)ne 0$, $operatorname{sgn}circ f$ is constant in a neighbourhood of $0$. Then from $(2)$ with $vapprox 0$, we see that $operatorname{sgn}circ f$ is locally constant, hence constant throughout, and finally must be $=1$, i.e.,
$$tag3f(x)>0qquad text{for all }xinBbb R.$$
Hence we can define $g(x)=ln f(x)$ for all $xin Bbb R$ and conclude from $(1)$ that $g$ is additive.
It is well-known that continuity at a single point (as here implied by the existence of $f'(0)$) implies that all solutions are of the form
$$ g(x)=cx.$$
Therefore
$$ f(x)=e^{cx}$$
for a suitable constant $c$.
It follows that $f$ is smooth and that
$$ f'(x)=cf(x)$$
so that
$$f'(10)=frac{f'(0)}{f(0)}f(10)=8. $$
$endgroup$
We need not assume that $f$ is differentiable or even continuous except at $0$.
With $x=frac{u+v}2$, $y=frac{u-v}2$, we obtain
$$tag1 f(u+v)=f(u)f(v)qquad text{for all }u,vinBbb R .$$
Then also
$$tag2operatorname{sgn}(f(u+v))= operatorname{sgn}(f(u))operatorname{sgn}(f(v))qquad text{for all }u,vinBbb R .$$
As $f$ is continuous at $0$ and $f(0)ne 0$, $operatorname{sgn}circ f$ is constant in a neighbourhood of $0$. Then from $(2)$ with $vapprox 0$, we see that $operatorname{sgn}circ f$ is locally constant, hence constant throughout, and finally must be $=1$, i.e.,
$$tag3f(x)>0qquad text{for all }xinBbb R.$$
Hence we can define $g(x)=ln f(x)$ for all $xin Bbb R$ and conclude from $(1)$ that $g$ is additive.
It is well-known that continuity at a single point (as here implied by the existence of $f'(0)$) implies that all solutions are of the form
$$ g(x)=cx.$$
Therefore
$$ f(x)=e^{cx}$$
for a suitable constant $c$.
It follows that $f$ is smooth and that
$$ f'(x)=cf(x)$$
so that
$$f'(10)=frac{f'(0)}{f(0)}f(10)=8. $$
answered Dec 7 '18 at 20:27
Hagen von EitzenHagen von Eitzen
279k23271501
279k23271501
add a comment |
add a comment |
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Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.
$endgroup$
– José Carlos Santos
Dec 7 '18 at 7:22