Compute $limlimits_{xto0} left(sin x + cos xright)^{1/x}$
$begingroup$
I hit a snag while solving exponential functions whose limits are given.
Question:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)$$
My Approach:
I am using the followin relation to solve the question of these type.
$$lim_{xto0} left(1 + xright)^left(1/xright) = e qquad(2)$$
But now how should i convert my above question so that i can apply the rule as mentioned in $(2)$.
Conclusion:
First of all help will be appreciated.
Second how to solve functions of such kind in a quick method.
Thanks,
P.S.(Feel free to edit my question if you find any errors or mistakes in my question)
limits
$endgroup$
add a comment |
$begingroup$
I hit a snag while solving exponential functions whose limits are given.
Question:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)$$
My Approach:
I am using the followin relation to solve the question of these type.
$$lim_{xto0} left(1 + xright)^left(1/xright) = e qquad(2)$$
But now how should i convert my above question so that i can apply the rule as mentioned in $(2)$.
Conclusion:
First of all help will be appreciated.
Second how to solve functions of such kind in a quick method.
Thanks,
P.S.(Feel free to edit my question if you find any errors or mistakes in my question)
limits
$endgroup$
$begingroup$
@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
@Did , please elaborate why would the LH step be absurd?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:24
2
$begingroup$
Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
$endgroup$
– Did
Dec 31 '18 at 12:29
add a comment |
$begingroup$
I hit a snag while solving exponential functions whose limits are given.
Question:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)$$
My Approach:
I am using the followin relation to solve the question of these type.
$$lim_{xto0} left(1 + xright)^left(1/xright) = e qquad(2)$$
But now how should i convert my above question so that i can apply the rule as mentioned in $(2)$.
Conclusion:
First of all help will be appreciated.
Second how to solve functions of such kind in a quick method.
Thanks,
P.S.(Feel free to edit my question if you find any errors or mistakes in my question)
limits
$endgroup$
I hit a snag while solving exponential functions whose limits are given.
Question:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)$$
My Approach:
I am using the followin relation to solve the question of these type.
$$lim_{xto0} left(1 + xright)^left(1/xright) = e qquad(2)$$
But now how should i convert my above question so that i can apply the rule as mentioned in $(2)$.
Conclusion:
First of all help will be appreciated.
Second how to solve functions of such kind in a quick method.
Thanks,
P.S.(Feel free to edit my question if you find any errors or mistakes in my question)
limits
limits
edited Dec 31 '18 at 14:17
Fortox
5718
5718
asked Dec 31 '18 at 12:04
GingitsuneGingitsune
247
247
$begingroup$
@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
@Did , please elaborate why would the LH step be absurd?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:24
2
$begingroup$
Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
$endgroup$
– Did
Dec 31 '18 at 12:29
add a comment |
$begingroup$
@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
@Did , please elaborate why would the LH step be absurd?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:24
2
$begingroup$
Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
$endgroup$
– Did
Dec 31 '18 at 12:29
$begingroup$
@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
@Did , please elaborate why would the LH step be absurd?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:24
$begingroup$
@Did , please elaborate why would the LH step be absurd?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:24
2
2
$begingroup$
Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
$endgroup$
– Did
Dec 31 '18 at 12:29
$begingroup$
Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
$endgroup$
– Did
Dec 31 '18 at 12:29
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
In this case the best idea is take logarithm, and then use De l'Hopital. $$lim_{x to 0} frac{ln (sin x + cos x)}{x} = lim_{x to 0} frac{cos x - sin x}{sin x + cos x} = 1$$ Hence the answer is $e^{1}=e$.
$endgroup$
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
add a comment |
$begingroup$
Alternatively, take $cos x$ out:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)=\
lim_{xto0} left(cos xright)^left(1/xright)cdot lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)}cdot color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)}=color{blue}1cdot color{red}e=e,$$
because using the relation you want:
$$color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)=\
lim_{xto0} left[left(1+left(-2sin^2 frac x2right)right)^frac{1}{-2sin^2 frac x2}right]^{frac{-2sin^2 frac x2}{x}}=e^0=1};\
color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
lim_{xto0} left[left(1+frac{sin x}{cos x}right)^{frac{cos x}{sin x}}right]^frac{sin x}{xcos x}=e^1=e}.$$
$endgroup$
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
add a comment |
$begingroup$
Another standard way works with enforcing a $color{red}{1}$ in the basis and considering the resulting exponent:
begin{eqnarray*} left(sin x + cos xright)^{1/x}
& = & left( color{red}{1} + (color{blue}{sin x + cos x - 1})right)^{1/x}\
& = & left (left( 1 + (color{blue}{sin x + cos x - 1})right)^{1/(color{blue}{sin x + cos x - 1})}right)^{frac{color{blue}{sin x + cos x - 1}}{x}} \
& stackrel {x to 0}{longrightarrow} & e^{lim_{xto 0}frac{sin x + cos x - 1}{x}}\
& = & e^{1+cos'(0)} = e \
end{eqnarray*}
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
In this case the best idea is take logarithm, and then use De l'Hopital. $$lim_{x to 0} frac{ln (sin x + cos x)}{x} = lim_{x to 0} frac{cos x - sin x}{sin x + cos x} = 1$$ Hence the answer is $e^{1}=e$.
$endgroup$
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
add a comment |
$begingroup$
In this case the best idea is take logarithm, and then use De l'Hopital. $$lim_{x to 0} frac{ln (sin x + cos x)}{x} = lim_{x to 0} frac{cos x - sin x}{sin x + cos x} = 1$$ Hence the answer is $e^{1}=e$.
$endgroup$
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
add a comment |
$begingroup$
In this case the best idea is take logarithm, and then use De l'Hopital. $$lim_{x to 0} frac{ln (sin x + cos x)}{x} = lim_{x to 0} frac{cos x - sin x}{sin x + cos x} = 1$$ Hence the answer is $e^{1}=e$.
$endgroup$
In this case the best idea is take logarithm, and then use De l'Hopital. $$lim_{x to 0} frac{ln (sin x + cos x)}{x} = lim_{x to 0} frac{cos x - sin x}{sin x + cos x} = 1$$ Hence the answer is $e^{1}=e$.
edited Dec 31 '18 at 12:40
answered Dec 31 '18 at 12:08
CrostulCrostul
28.3k22352
28.3k22352
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
add a comment |
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
$begingroup$
See comment on main.
$endgroup$
– Did
Dec 31 '18 at 12:10
1
1
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
$begingroup$
It isn't $e^{-1}$ anyway
$endgroup$
– Kenny Lau
Dec 31 '18 at 12:19
2
2
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
$begingroup$
@Crostul , When apply the $lim_{xto0}$ shouldn't the answer be $1$ ?
$endgroup$
– Gingitsune
Dec 31 '18 at 12:21
add a comment |
$begingroup$
Alternatively, take $cos x$ out:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)=\
lim_{xto0} left(cos xright)^left(1/xright)cdot lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)}cdot color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)}=color{blue}1cdot color{red}e=e,$$
because using the relation you want:
$$color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)=\
lim_{xto0} left[left(1+left(-2sin^2 frac x2right)right)^frac{1}{-2sin^2 frac x2}right]^{frac{-2sin^2 frac x2}{x}}=e^0=1};\
color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
lim_{xto0} left[left(1+frac{sin x}{cos x}right)^{frac{cos x}{sin x}}right]^frac{sin x}{xcos x}=e^1=e}.$$
$endgroup$
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
add a comment |
$begingroup$
Alternatively, take $cos x$ out:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)=\
lim_{xto0} left(cos xright)^left(1/xright)cdot lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)}cdot color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)}=color{blue}1cdot color{red}e=e,$$
because using the relation you want:
$$color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)=\
lim_{xto0} left[left(1+left(-2sin^2 frac x2right)right)^frac{1}{-2sin^2 frac x2}right]^{frac{-2sin^2 frac x2}{x}}=e^0=1};\
color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
lim_{xto0} left[left(1+frac{sin x}{cos x}right)^{frac{cos x}{sin x}}right]^frac{sin x}{xcos x}=e^1=e}.$$
$endgroup$
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
add a comment |
$begingroup$
Alternatively, take $cos x$ out:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)=\
lim_{xto0} left(cos xright)^left(1/xright)cdot lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)}cdot color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)}=color{blue}1cdot color{red}e=e,$$
because using the relation you want:
$$color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)=\
lim_{xto0} left[left(1+left(-2sin^2 frac x2right)right)^frac{1}{-2sin^2 frac x2}right]^{frac{-2sin^2 frac x2}{x}}=e^0=1};\
color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
lim_{xto0} left[left(1+frac{sin x}{cos x}right)^{frac{cos x}{sin x}}right]^frac{sin x}{xcos x}=e^1=e}.$$
$endgroup$
Alternatively, take $cos x$ out:
$$lim_{xto0} left(sin x + cos xright)^left(1/xright)=\
lim_{xto0} left(cos xright)^left(1/xright)cdot lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)}cdot color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)}=color{blue}1cdot color{red}e=e,$$
because using the relation you want:
$$color{blue}{lim_{xto0} left(1-2sin^2 frac x2right)^left(1/xright)=\
lim_{xto0} left[left(1+left(-2sin^2 frac x2right)right)^frac{1}{-2sin^2 frac x2}right]^{frac{-2sin^2 frac x2}{x}}=e^0=1};\
color{red}{lim_{xto0} left(1+frac{sin x}{cos x}right)^left(1/xright)=\
lim_{xto0} left[left(1+frac{sin x}{cos x}right)^{frac{cos x}{sin x}}right]^frac{sin x}{xcos x}=e^1=e}.$$
answered Jan 1 at 10:05
farruhotafarruhota
22.5k2942
22.5k2942
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
add a comment |
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
$begingroup$
thanks you helped me find one more way to approach questions of this kind
$endgroup$
– Gingitsune
Jan 1 at 12:06
add a comment |
$begingroup$
Another standard way works with enforcing a $color{red}{1}$ in the basis and considering the resulting exponent:
begin{eqnarray*} left(sin x + cos xright)^{1/x}
& = & left( color{red}{1} + (color{blue}{sin x + cos x - 1})right)^{1/x}\
& = & left (left( 1 + (color{blue}{sin x + cos x - 1})right)^{1/(color{blue}{sin x + cos x - 1})}right)^{frac{color{blue}{sin x + cos x - 1}}{x}} \
& stackrel {x to 0}{longrightarrow} & e^{lim_{xto 0}frac{sin x + cos x - 1}{x}}\
& = & e^{1+cos'(0)} = e \
end{eqnarray*}
$endgroup$
add a comment |
$begingroup$
Another standard way works with enforcing a $color{red}{1}$ in the basis and considering the resulting exponent:
begin{eqnarray*} left(sin x + cos xright)^{1/x}
& = & left( color{red}{1} + (color{blue}{sin x + cos x - 1})right)^{1/x}\
& = & left (left( 1 + (color{blue}{sin x + cos x - 1})right)^{1/(color{blue}{sin x + cos x - 1})}right)^{frac{color{blue}{sin x + cos x - 1}}{x}} \
& stackrel {x to 0}{longrightarrow} & e^{lim_{xto 0}frac{sin x + cos x - 1}{x}}\
& = & e^{1+cos'(0)} = e \
end{eqnarray*}
$endgroup$
add a comment |
$begingroup$
Another standard way works with enforcing a $color{red}{1}$ in the basis and considering the resulting exponent:
begin{eqnarray*} left(sin x + cos xright)^{1/x}
& = & left( color{red}{1} + (color{blue}{sin x + cos x - 1})right)^{1/x}\
& = & left (left( 1 + (color{blue}{sin x + cos x - 1})right)^{1/(color{blue}{sin x + cos x - 1})}right)^{frac{color{blue}{sin x + cos x - 1}}{x}} \
& stackrel {x to 0}{longrightarrow} & e^{lim_{xto 0}frac{sin x + cos x - 1}{x}}\
& = & e^{1+cos'(0)} = e \
end{eqnarray*}
$endgroup$
Another standard way works with enforcing a $color{red}{1}$ in the basis and considering the resulting exponent:
begin{eqnarray*} left(sin x + cos xright)^{1/x}
& = & left( color{red}{1} + (color{blue}{sin x + cos x - 1})right)^{1/x}\
& = & left (left( 1 + (color{blue}{sin x + cos x - 1})right)^{1/(color{blue}{sin x + cos x - 1})}right)^{frac{color{blue}{sin x + cos x - 1}}{x}} \
& stackrel {x to 0}{longrightarrow} & e^{lim_{xto 0}frac{sin x + cos x - 1}{x}}\
& = & e^{1+cos'(0)} = e \
end{eqnarray*}
answered Jan 1 at 6:11
trancelocationtrancelocation
14.7k1929
14.7k1929
add a comment |
add a comment |
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@Crostul The LH step in this case would be absurd, as in every situation where one is after a limit $limlimits_{xto0}g(x)/x$ with $g$ differentiable at $0$ and $g(0)=0$.
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– Did
Dec 31 '18 at 12:10
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@Did , please elaborate why would the LH step be absurd?
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– Gingitsune
Dec 31 '18 at 12:24
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Because using LH to find $limlimits_{xto0} g(x)/h(x)$ when $g(0)=h(0)=0$ requires to compute $g'(0)$ and $h'(0)$. But, in the case when $h(x)=x$, one already knows that $limlimits_{xto0}g(x)/x=g'(0)$ by definition of the derivative of $g$ since $g(x)/x=(g(x)-g(0))/(x-0)$. So, invoking LH here amounts to a beautifully (?) absurd logical circle.
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– Did
Dec 31 '18 at 12:29