How do I compute $lim_{x to a}(a-x)tanleft(frac{πx}{2a}right)$
$begingroup$
Evaluate
$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$
I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.
limits limits-without-lhopital
$endgroup$
add a comment |
$begingroup$
Evaluate
$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$
I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.
limits limits-without-lhopital
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– Robert Z
Dec 12 '18 at 16:25
add a comment |
$begingroup$
Evaluate
$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$
I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.
limits limits-without-lhopital
$endgroup$
Evaluate
$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$
I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.
limits limits-without-lhopital
limits limits-without-lhopital
edited Dec 11 '18 at 18:54
Lorenzo B.
1,8602520
1,8602520
asked Dec 11 '18 at 15:53
S.NepS.Nep
776
776
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– Robert Z
Dec 12 '18 at 16:25
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– Robert Z
Dec 12 '18 at 16:25
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– Robert Z
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– Robert Z
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4 Answers
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$begingroup$
As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$
Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$
Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$
We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$
$endgroup$
add a comment |
$begingroup$
It does lead to a solution, keep going:
$$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
&=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
&=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
&=frac{2a}{pi},end{align}$$
where it was used:
$$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$
$endgroup$
add a comment |
$begingroup$
Let $a-x=y$
For $ane0,$
$$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$
Use $lim_{hto0}dfrac{sin h}h=1$
$endgroup$
add a comment |
$begingroup$
$a not =0$.
Set $y =(πx)/(2a).$
Then $y rightarrow π/2.$
$((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$
Recall $sin (π/2-y) =cos y.$
Then
$(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.
Can you take the limit $y rightarrow π/2?$
P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$
Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$
Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$
We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$
$endgroup$
add a comment |
$begingroup$
As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$
Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$
Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$
We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$
$endgroup$
add a comment |
$begingroup$
As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$
Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$
Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$
We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$
$endgroup$
As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$
Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$
Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$
We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$
answered Dec 13 '18 at 10:38
romanroman
2,28421224
2,28421224
add a comment |
add a comment |
$begingroup$
It does lead to a solution, keep going:
$$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
&=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
&=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
&=frac{2a}{pi},end{align}$$
where it was used:
$$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$
$endgroup$
add a comment |
$begingroup$
It does lead to a solution, keep going:
$$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
&=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
&=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
&=frac{2a}{pi},end{align}$$
where it was used:
$$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$
$endgroup$
add a comment |
$begingroup$
It does lead to a solution, keep going:
$$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
&=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
&=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
&=frac{2a}{pi},end{align}$$
where it was used:
$$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$
$endgroup$
It does lead to a solution, keep going:
$$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
&=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
&=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
&=frac{2a}{pi},end{align}$$
where it was used:
$$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$
answered Dec 11 '18 at 18:26
farruhotafarruhota
20.5k2739
20.5k2739
add a comment |
add a comment |
$begingroup$
Let $a-x=y$
For $ane0,$
$$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$
Use $lim_{hto0}dfrac{sin h}h=1$
$endgroup$
add a comment |
$begingroup$
Let $a-x=y$
For $ane0,$
$$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$
Use $lim_{hto0}dfrac{sin h}h=1$
$endgroup$
add a comment |
$begingroup$
Let $a-x=y$
For $ane0,$
$$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$
Use $lim_{hto0}dfrac{sin h}h=1$
$endgroup$
Let $a-x=y$
For $ane0,$
$$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$
Use $lim_{hto0}dfrac{sin h}h=1$
answered Dec 11 '18 at 15:58
lab bhattacharjeelab bhattacharjee
226k15157275
226k15157275
add a comment |
add a comment |
$begingroup$
$a not =0$.
Set $y =(πx)/(2a).$
Then $y rightarrow π/2.$
$((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$
Recall $sin (π/2-y) =cos y.$
Then
$(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.
Can you take the limit $y rightarrow π/2?$
P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.
$endgroup$
add a comment |
$begingroup$
$a not =0$.
Set $y =(πx)/(2a).$
Then $y rightarrow π/2.$
$((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$
Recall $sin (π/2-y) =cos y.$
Then
$(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.
Can you take the limit $y rightarrow π/2?$
P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.
$endgroup$
add a comment |
$begingroup$
$a not =0$.
Set $y =(πx)/(2a).$
Then $y rightarrow π/2.$
$((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$
Recall $sin (π/2-y) =cos y.$
Then
$(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.
Can you take the limit $y rightarrow π/2?$
P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.
$endgroup$
$a not =0$.
Set $y =(πx)/(2a).$
Then $y rightarrow π/2.$
$((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$
Recall $sin (π/2-y) =cos y.$
Then
$(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.
Can you take the limit $y rightarrow π/2?$
P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.
edited Dec 13 '18 at 8:49
answered Dec 11 '18 at 16:25
Peter SzilasPeter Szilas
11.4k2822
11.4k2822
add a comment |
add a comment |
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– Robert Z
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