Why can't I use the formula for $f(x)=x^x$ to derive $(frac{x}{x+1})^x$?
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Why can't I use the formula $y=x^x$, $dot {y}=x^x(ln x+1)$ to derive $(frac{x}{x+1})^x$?
I've tried and it doesn't get me the correct result. Have I done any mistake?
$$Bigl[Bigl(frac{x}{x+1}Bigr)^xBigr]'=(frac{x}{x+1})^xBigl[lnBigl(frac{x}{x+1}Bigr)+1Bigr]Bigl[frac{1+x-x}{(x+1)^2}Bigr]=frac{bigl(frac{x}{x+1}bigr)^xBigl[lnbigl(frac{x}{x+1}bigr)+1Bigr]}{(1+x)^2}$$
If not, please explain why I can't use the formula.
derivatives
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add a comment |
$begingroup$
Why can't I use the formula $y=x^x$, $dot {y}=x^x(ln x+1)$ to derive $(frac{x}{x+1})^x$?
I've tried and it doesn't get me the correct result. Have I done any mistake?
$$Bigl[Bigl(frac{x}{x+1}Bigr)^xBigr]'=(frac{x}{x+1})^xBigl[lnBigl(frac{x}{x+1}Bigr)+1Bigr]Bigl[frac{1+x-x}{(x+1)^2}Bigr]=frac{bigl(frac{x}{x+1}bigr)^xBigl[lnbigl(frac{x}{x+1}bigr)+1Bigr]}{(1+x)^2}$$
If not, please explain why I can't use the formula.
derivatives
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2
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Simple: you need the quotient and chain rules.
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– Sean Roberson
Jan 10 at 19:10
add a comment |
$begingroup$
Why can't I use the formula $y=x^x$, $dot {y}=x^x(ln x+1)$ to derive $(frac{x}{x+1})^x$?
I've tried and it doesn't get me the correct result. Have I done any mistake?
$$Bigl[Bigl(frac{x}{x+1}Bigr)^xBigr]'=(frac{x}{x+1})^xBigl[lnBigl(frac{x}{x+1}Bigr)+1Bigr]Bigl[frac{1+x-x}{(x+1)^2}Bigr]=frac{bigl(frac{x}{x+1}bigr)^xBigl[lnbigl(frac{x}{x+1}bigr)+1Bigr]}{(1+x)^2}$$
If not, please explain why I can't use the formula.
derivatives
$endgroup$
Why can't I use the formula $y=x^x$, $dot {y}=x^x(ln x+1)$ to derive $(frac{x}{x+1})^x$?
I've tried and it doesn't get me the correct result. Have I done any mistake?
$$Bigl[Bigl(frac{x}{x+1}Bigr)^xBigr]'=(frac{x}{x+1})^xBigl[lnBigl(frac{x}{x+1}Bigr)+1Bigr]Bigl[frac{1+x-x}{(x+1)^2}Bigr]=frac{bigl(frac{x}{x+1}bigr)^xBigl[lnbigl(frac{x}{x+1}bigr)+1Bigr]}{(1+x)^2}$$
If not, please explain why I can't use the formula.
derivatives
derivatives
edited Jan 10 at 19:14
Bernard
122k740116
122k740116
asked Jan 10 at 19:10
Lauren SinLauren Sin
1055
1055
2
$begingroup$
Simple: you need the quotient and chain rules.
$endgroup$
– Sean Roberson
Jan 10 at 19:10
add a comment |
2
$begingroup$
Simple: you need the quotient and chain rules.
$endgroup$
– Sean Roberson
Jan 10 at 19:10
2
2
$begingroup$
Simple: you need the quotient and chain rules.
$endgroup$
– Sean Roberson
Jan 10 at 19:10
$begingroup$
Simple: you need the quotient and chain rules.
$endgroup$
– Sean Roberson
Jan 10 at 19:10
add a comment |
5 Answers
5
active
oldest
votes
$begingroup$
Hint:
Your error comes from the fact that in your formula, the variable and the exponent have to be identical. I suggest using the logarithmic derivative: if $f(x)=bigl(u(x)bigr)^x$
$$frac{f'(x)}{f(x)}=lnbigl(u(x)bigr)
+x,frac{u'(x)}{u(x)}.$$
$endgroup$
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
add a comment |
$begingroup$
Define $z:=frac{x}{x+1}$. Your claim is that $frac{d}{dx}(z^x)=frac{d}{dz}(z^z)cdotfrac{dz}{dx}$, which is clearly wrong because the right-hand side is actually $frac{d}{dx}(z^z)$. I'll leave you to solve the problem properly, the very same way you'd differentiate $x^x$ in the first place: by logarithmic differentiation.
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add a comment |
$begingroup$
To apply the chain rule directly, you need something of the form $f(g(x))$; in your case that would be $g(x)^{g(x)}$, but that's not what you have: you have $g(x)^x$.
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add a comment |
$begingroup$
Hint: I have got this here :$${frac {1}{x+1} left( {frac {x}{x+1}} right) ^{x} left( ln
left( {frac {x}{x+1}} right) x+ln left( {frac {x}{x+1}}
right) +1 right) }
$$
Taking the logarithm on both sides we get
$$ln(y)=x(ln(x)-ln(x+1))$$ and differentiating with respect to $x$
$$frac{1}{y}y'=ln(x)-ln(x+1)+xleft(frac{1}{x}-frac{1}{x+1}right)$$
$endgroup$
add a comment |
$begingroup$
What you have used is that $$left [g(x)^xright]'=g'(x)cdot g(x)^xcdot ln g(x)+1$$which doesn't generally hold. To solve this problem just simply write$$ln f(x)=xln x-xln(x+1)$$whose first-order differentiation leads to $${f'(x)over f(x)}=1+ln x-ln (x+1)-{xover x+1}$$from which we obtain$$f'(x)=left({xover x+1}right)^xcdot left({1over x+1}+ln{xover x+1}right)$$
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add a comment |
Your Answer
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5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint:
Your error comes from the fact that in your formula, the variable and the exponent have to be identical. I suggest using the logarithmic derivative: if $f(x)=bigl(u(x)bigr)^x$
$$frac{f'(x)}{f(x)}=lnbigl(u(x)bigr)
+x,frac{u'(x)}{u(x)}.$$
$endgroup$
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
add a comment |
$begingroup$
Hint:
Your error comes from the fact that in your formula, the variable and the exponent have to be identical. I suggest using the logarithmic derivative: if $f(x)=bigl(u(x)bigr)^x$
$$frac{f'(x)}{f(x)}=lnbigl(u(x)bigr)
+x,frac{u'(x)}{u(x)}.$$
$endgroup$
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
add a comment |
$begingroup$
Hint:
Your error comes from the fact that in your formula, the variable and the exponent have to be identical. I suggest using the logarithmic derivative: if $f(x)=bigl(u(x)bigr)^x$
$$frac{f'(x)}{f(x)}=lnbigl(u(x)bigr)
+x,frac{u'(x)}{u(x)}.$$
$endgroup$
Hint:
Your error comes from the fact that in your formula, the variable and the exponent have to be identical. I suggest using the logarithmic derivative: if $f(x)=bigl(u(x)bigr)^x$
$$frac{f'(x)}{f(x)}=lnbigl(u(x)bigr)
+x,frac{u'(x)}{u(x)}.$$
edited Jan 10 at 20:39
answered Jan 10 at 19:32
BernardBernard
122k740116
122k740116
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
add a comment |
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
$begingroup$
Sorry for the typo – I forgot a factor $x$. The formula is fixed now.
$endgroup$
– Bernard
Jan 10 at 20:40
add a comment |
$begingroup$
Define $z:=frac{x}{x+1}$. Your claim is that $frac{d}{dx}(z^x)=frac{d}{dz}(z^z)cdotfrac{dz}{dx}$, which is clearly wrong because the right-hand side is actually $frac{d}{dx}(z^z)$. I'll leave you to solve the problem properly, the very same way you'd differentiate $x^x$ in the first place: by logarithmic differentiation.
$endgroup$
add a comment |
$begingroup$
Define $z:=frac{x}{x+1}$. Your claim is that $frac{d}{dx}(z^x)=frac{d}{dz}(z^z)cdotfrac{dz}{dx}$, which is clearly wrong because the right-hand side is actually $frac{d}{dx}(z^z)$. I'll leave you to solve the problem properly, the very same way you'd differentiate $x^x$ in the first place: by logarithmic differentiation.
$endgroup$
add a comment |
$begingroup$
Define $z:=frac{x}{x+1}$. Your claim is that $frac{d}{dx}(z^x)=frac{d}{dz}(z^z)cdotfrac{dz}{dx}$, which is clearly wrong because the right-hand side is actually $frac{d}{dx}(z^z)$. I'll leave you to solve the problem properly, the very same way you'd differentiate $x^x$ in the first place: by logarithmic differentiation.
$endgroup$
Define $z:=frac{x}{x+1}$. Your claim is that $frac{d}{dx}(z^x)=frac{d}{dz}(z^z)cdotfrac{dz}{dx}$, which is clearly wrong because the right-hand side is actually $frac{d}{dx}(z^z)$. I'll leave you to solve the problem properly, the very same way you'd differentiate $x^x$ in the first place: by logarithmic differentiation.
answered Jan 10 at 19:16
J.G.J.G.
28.7k22845
28.7k22845
add a comment |
add a comment |
$begingroup$
To apply the chain rule directly, you need something of the form $f(g(x))$; in your case that would be $g(x)^{g(x)}$, but that's not what you have: you have $g(x)^x$.
$endgroup$
add a comment |
$begingroup$
To apply the chain rule directly, you need something of the form $f(g(x))$; in your case that would be $g(x)^{g(x)}$, but that's not what you have: you have $g(x)^x$.
$endgroup$
add a comment |
$begingroup$
To apply the chain rule directly, you need something of the form $f(g(x))$; in your case that would be $g(x)^{g(x)}$, but that's not what you have: you have $g(x)^x$.
$endgroup$
To apply the chain rule directly, you need something of the form $f(g(x))$; in your case that would be $g(x)^{g(x)}$, but that's not what you have: you have $g(x)^x$.
answered Jan 10 at 19:22
Martin ArgeramiMartin Argerami
128k1183183
128k1183183
add a comment |
add a comment |
$begingroup$
Hint: I have got this here :$${frac {1}{x+1} left( {frac {x}{x+1}} right) ^{x} left( ln
left( {frac {x}{x+1}} right) x+ln left( {frac {x}{x+1}}
right) +1 right) }
$$
Taking the logarithm on both sides we get
$$ln(y)=x(ln(x)-ln(x+1))$$ and differentiating with respect to $x$
$$frac{1}{y}y'=ln(x)-ln(x+1)+xleft(frac{1}{x}-frac{1}{x+1}right)$$
$endgroup$
add a comment |
$begingroup$
Hint: I have got this here :$${frac {1}{x+1} left( {frac {x}{x+1}} right) ^{x} left( ln
left( {frac {x}{x+1}} right) x+ln left( {frac {x}{x+1}}
right) +1 right) }
$$
Taking the logarithm on both sides we get
$$ln(y)=x(ln(x)-ln(x+1))$$ and differentiating with respect to $x$
$$frac{1}{y}y'=ln(x)-ln(x+1)+xleft(frac{1}{x}-frac{1}{x+1}right)$$
$endgroup$
add a comment |
$begingroup$
Hint: I have got this here :$${frac {1}{x+1} left( {frac {x}{x+1}} right) ^{x} left( ln
left( {frac {x}{x+1}} right) x+ln left( {frac {x}{x+1}}
right) +1 right) }
$$
Taking the logarithm on both sides we get
$$ln(y)=x(ln(x)-ln(x+1))$$ and differentiating with respect to $x$
$$frac{1}{y}y'=ln(x)-ln(x+1)+xleft(frac{1}{x}-frac{1}{x+1}right)$$
$endgroup$
Hint: I have got this here :$${frac {1}{x+1} left( {frac {x}{x+1}} right) ^{x} left( ln
left( {frac {x}{x+1}} right) x+ln left( {frac {x}{x+1}}
right) +1 right) }
$$
Taking the logarithm on both sides we get
$$ln(y)=x(ln(x)-ln(x+1))$$ and differentiating with respect to $x$
$$frac{1}{y}y'=ln(x)-ln(x+1)+xleft(frac{1}{x}-frac{1}{x+1}right)$$
edited Jan 10 at 19:34
answered Jan 10 at 19:14
Dr. Sonnhard GraubnerDr. Sonnhard Graubner
76.8k42866
76.8k42866
add a comment |
add a comment |
$begingroup$
What you have used is that $$left [g(x)^xright]'=g'(x)cdot g(x)^xcdot ln g(x)+1$$which doesn't generally hold. To solve this problem just simply write$$ln f(x)=xln x-xln(x+1)$$whose first-order differentiation leads to $${f'(x)over f(x)}=1+ln x-ln (x+1)-{xover x+1}$$from which we obtain$$f'(x)=left({xover x+1}right)^xcdot left({1over x+1}+ln{xover x+1}right)$$
$endgroup$
add a comment |
$begingroup$
What you have used is that $$left [g(x)^xright]'=g'(x)cdot g(x)^xcdot ln g(x)+1$$which doesn't generally hold. To solve this problem just simply write$$ln f(x)=xln x-xln(x+1)$$whose first-order differentiation leads to $${f'(x)over f(x)}=1+ln x-ln (x+1)-{xover x+1}$$from which we obtain$$f'(x)=left({xover x+1}right)^xcdot left({1over x+1}+ln{xover x+1}right)$$
$endgroup$
add a comment |
$begingroup$
What you have used is that $$left [g(x)^xright]'=g'(x)cdot g(x)^xcdot ln g(x)+1$$which doesn't generally hold. To solve this problem just simply write$$ln f(x)=xln x-xln(x+1)$$whose first-order differentiation leads to $${f'(x)over f(x)}=1+ln x-ln (x+1)-{xover x+1}$$from which we obtain$$f'(x)=left({xover x+1}right)^xcdot left({1over x+1}+ln{xover x+1}right)$$
$endgroup$
What you have used is that $$left [g(x)^xright]'=g'(x)cdot g(x)^xcdot ln g(x)+1$$which doesn't generally hold. To solve this problem just simply write$$ln f(x)=xln x-xln(x+1)$$whose first-order differentiation leads to $${f'(x)over f(x)}=1+ln x-ln (x+1)-{xover x+1}$$from which we obtain$$f'(x)=left({xover x+1}right)^xcdot left({1over x+1}+ln{xover x+1}right)$$
answered Jan 10 at 20:19
Mostafa AyazMostafa Ayaz
15.7k3939
15.7k3939
add a comment |
add a comment |
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$begingroup$
Simple: you need the quotient and chain rules.
$endgroup$
– Sean Roberson
Jan 10 at 19:10