Calculate $limlimits_{ xto + infty}xcdot sin(sqrt{x^{2}+3}-sqrt{x^{2}+2})$
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I know that $$limlimits_{ xto + infty}xcdot sin(sqrt{x^{2}+3}-sqrt{x^{2}+2})\=limlimits_{ xto + infty}xcdot sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right).$$ If $x rightarrow + infty$, then $sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right)rightarrow sin0 $. However I have also $x$ before $sin x$ and I don't know how to calculate it.
real-analysis limits
$endgroup$
add a comment |
$begingroup$
I know that $$limlimits_{ xto + infty}xcdot sin(sqrt{x^{2}+3}-sqrt{x^{2}+2})\=limlimits_{ xto + infty}xcdot sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right).$$ If $x rightarrow + infty$, then $sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right)rightarrow sin0 $. However I have also $x$ before $sin x$ and I don't know how to calculate it.
real-analysis limits
$endgroup$
add a comment |
$begingroup$
I know that $$limlimits_{ xto + infty}xcdot sin(sqrt{x^{2}+3}-sqrt{x^{2}+2})\=limlimits_{ xto + infty}xcdot sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right).$$ If $x rightarrow + infty$, then $sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right)rightarrow sin0 $. However I have also $x$ before $sin x$ and I don't know how to calculate it.
real-analysis limits
$endgroup$
I know that $$limlimits_{ xto + infty}xcdot sin(sqrt{x^{2}+3}-sqrt{x^{2}+2})\=limlimits_{ xto + infty}xcdot sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right).$$ If $x rightarrow + infty$, then $sinleft(frac{1}{sqrt{x^{2}+3}+sqrt{x^{2}+2}}right)rightarrow sin0 $. However I have also $x$ before $sin x$ and I don't know how to calculate it.
real-analysis limits
real-analysis limits
edited Dec 31 '18 at 13:52
user376343
3,9584829
3,9584829
asked Dec 31 '18 at 13:23
MP3129MP3129
607110
607110
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Letting $h=frac1x$:
$$begin{array}{cl}
&displaystyle lim_{x to infty} x sin left( sqrt{x^2+3} - sqrt{x^2+2} right) \
=&displaystyle lim_{x to infty} x sin left( frac 1 {sqrt{x^2+3} + sqrt{x^2+2} } right) \
=&displaystyle lim_{h to 0^+} frac1h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle lim_{h to 0^+} frac 1 {sqrt{1+3h^2} + sqrt{1+2h^2}} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times lim_{h to 0^+} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times 1 \
=&dfrac12
end{array}$$
$endgroup$
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
add a comment |
$begingroup$
begin{align}
lim_{x to infty} x cdot left( sin left( sqrt{x^2+3}-sqrt{x^2+2}right)right)&=lim_{x to infty} x cdot sin left( frac{1}{sqrt{x^2+3}+sqrt{x^2+2}}right)\
&=lim_{x to infty} frac{x}{sqrt{x^2+3}+sqrt{x^2+2}}\
&=lim_{x to infty} frac{1}{sqrt{1+frac{3}{x^2}}+sqrt{1+frac{2}{x^2}}}\
&= frac12
end{align}
$endgroup$
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
add a comment |
$begingroup$
The ordo calculus is very useful for such problems. Sometimes you do not need the full power of Taylor series, only the first couple of coefficients.
$sqrt{x^2+3}- sqrt{x^2+3} = x(sqrt{1+3/x^2}- sqrt{1+2/x^2})=$
$= x(1+frac{1}{2}cdot(3/x^2)+O(1/x^4)- 1-frac{1}{2}cdot(2/x^2)+O(1/x^4))= frac{1}{2x}+O(1/x^3)$.
Now $sin(frac{1}{2x}+O(1/x^3))= frac{1}{2x}+O(1/x^2)$ as $xrightarrow infty$, thus the expression is $frac{1}{2}+ O(frac{1}{x})$.
$endgroup$
add a comment |
Your Answer
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3 Answers
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active
oldest
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3 Answers
3
active
oldest
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active
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$begingroup$
Letting $h=frac1x$:
$$begin{array}{cl}
&displaystyle lim_{x to infty} x sin left( sqrt{x^2+3} - sqrt{x^2+2} right) \
=&displaystyle lim_{x to infty} x sin left( frac 1 {sqrt{x^2+3} + sqrt{x^2+2} } right) \
=&displaystyle lim_{h to 0^+} frac1h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle lim_{h to 0^+} frac 1 {sqrt{1+3h^2} + sqrt{1+2h^2}} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times lim_{h to 0^+} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times 1 \
=&dfrac12
end{array}$$
$endgroup$
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
add a comment |
$begingroup$
Letting $h=frac1x$:
$$begin{array}{cl}
&displaystyle lim_{x to infty} x sin left( sqrt{x^2+3} - sqrt{x^2+2} right) \
=&displaystyle lim_{x to infty} x sin left( frac 1 {sqrt{x^2+3} + sqrt{x^2+2} } right) \
=&displaystyle lim_{h to 0^+} frac1h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle lim_{h to 0^+} frac 1 {sqrt{1+3h^2} + sqrt{1+2h^2}} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times lim_{h to 0^+} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times 1 \
=&dfrac12
end{array}$$
$endgroup$
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
add a comment |
$begingroup$
Letting $h=frac1x$:
$$begin{array}{cl}
&displaystyle lim_{x to infty} x sin left( sqrt{x^2+3} - sqrt{x^2+2} right) \
=&displaystyle lim_{x to infty} x sin left( frac 1 {sqrt{x^2+3} + sqrt{x^2+2} } right) \
=&displaystyle lim_{h to 0^+} frac1h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle lim_{h to 0^+} frac 1 {sqrt{1+3h^2} + sqrt{1+2h^2}} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times lim_{h to 0^+} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times 1 \
=&dfrac12
end{array}$$
$endgroup$
Letting $h=frac1x$:
$$begin{array}{cl}
&displaystyle lim_{x to infty} x sin left( sqrt{x^2+3} - sqrt{x^2+2} right) \
=&displaystyle lim_{x to infty} x sin left( frac 1 {sqrt{x^2+3} + sqrt{x^2+2} } right) \
=&displaystyle lim_{h to 0^+} frac1h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle lim_{h to 0^+} frac 1 {sqrt{1+3h^2} + sqrt{1+2h^2}} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times lim_{h to 0^+} frac {sqrt{1+3h^2} + sqrt{1+2h^2}} h sin left( frac h {sqrt{1+3h^2} + sqrt{1+2h^2} } right) \
=&displaystyle frac12 times 1 \
=&dfrac12
end{array}$$
edited Dec 31 '18 at 13:37
answered Dec 31 '18 at 13:30
Kenny LauKenny Lau
20k2160
20k2160
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
add a comment |
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Actually the searched limit is equal to $$frac{1}{2}$$
$endgroup$
– Dr. Sonnhard Graubner
Dec 31 '18 at 13:36
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
$begingroup$
Fixed.${ }$${ }$
$endgroup$
– Kenny Lau
Dec 31 '18 at 13:37
add a comment |
$begingroup$
begin{align}
lim_{x to infty} x cdot left( sin left( sqrt{x^2+3}-sqrt{x^2+2}right)right)&=lim_{x to infty} x cdot sin left( frac{1}{sqrt{x^2+3}+sqrt{x^2+2}}right)\
&=lim_{x to infty} frac{x}{sqrt{x^2+3}+sqrt{x^2+2}}\
&=lim_{x to infty} frac{1}{sqrt{1+frac{3}{x^2}}+sqrt{1+frac{2}{x^2}}}\
&= frac12
end{align}
$endgroup$
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
add a comment |
$begingroup$
begin{align}
lim_{x to infty} x cdot left( sin left( sqrt{x^2+3}-sqrt{x^2+2}right)right)&=lim_{x to infty} x cdot sin left( frac{1}{sqrt{x^2+3}+sqrt{x^2+2}}right)\
&=lim_{x to infty} frac{x}{sqrt{x^2+3}+sqrt{x^2+2}}\
&=lim_{x to infty} frac{1}{sqrt{1+frac{3}{x^2}}+sqrt{1+frac{2}{x^2}}}\
&= frac12
end{align}
$endgroup$
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
add a comment |
$begingroup$
begin{align}
lim_{x to infty} x cdot left( sin left( sqrt{x^2+3}-sqrt{x^2+2}right)right)&=lim_{x to infty} x cdot sin left( frac{1}{sqrt{x^2+3}+sqrt{x^2+2}}right)\
&=lim_{x to infty} frac{x}{sqrt{x^2+3}+sqrt{x^2+2}}\
&=lim_{x to infty} frac{1}{sqrt{1+frac{3}{x^2}}+sqrt{1+frac{2}{x^2}}}\
&= frac12
end{align}
$endgroup$
begin{align}
lim_{x to infty} x cdot left( sin left( sqrt{x^2+3}-sqrt{x^2+2}right)right)&=lim_{x to infty} x cdot sin left( frac{1}{sqrt{x^2+3}+sqrt{x^2+2}}right)\
&=lim_{x to infty} frac{x}{sqrt{x^2+3}+sqrt{x^2+2}}\
&=lim_{x to infty} frac{1}{sqrt{1+frac{3}{x^2}}+sqrt{1+frac{2}{x^2}}}\
&= frac12
end{align}
answered Dec 31 '18 at 13:37
Siong Thye GohSiong Thye Goh
103k1468119
103k1468119
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
add a comment |
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
$begingroup$
You use the assumption that d.d.d. x $sinx = x$, I understand correctly?
$endgroup$
– MP3129
Dec 31 '18 at 14:02
2
2
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
$begingroup$
not sure what do those $d$ mean.The main trick is $lim_{h to 0^+} frac{sin h}h=1$, hence that is why I can remove the sine.
$endgroup$
– Siong Thye Goh
Dec 31 '18 at 14:07
add a comment |
$begingroup$
The ordo calculus is very useful for such problems. Sometimes you do not need the full power of Taylor series, only the first couple of coefficients.
$sqrt{x^2+3}- sqrt{x^2+3} = x(sqrt{1+3/x^2}- sqrt{1+2/x^2})=$
$= x(1+frac{1}{2}cdot(3/x^2)+O(1/x^4)- 1-frac{1}{2}cdot(2/x^2)+O(1/x^4))= frac{1}{2x}+O(1/x^3)$.
Now $sin(frac{1}{2x}+O(1/x^3))= frac{1}{2x}+O(1/x^2)$ as $xrightarrow infty$, thus the expression is $frac{1}{2}+ O(frac{1}{x})$.
$endgroup$
add a comment |
$begingroup$
The ordo calculus is very useful for such problems. Sometimes you do not need the full power of Taylor series, only the first couple of coefficients.
$sqrt{x^2+3}- sqrt{x^2+3} = x(sqrt{1+3/x^2}- sqrt{1+2/x^2})=$
$= x(1+frac{1}{2}cdot(3/x^2)+O(1/x^4)- 1-frac{1}{2}cdot(2/x^2)+O(1/x^4))= frac{1}{2x}+O(1/x^3)$.
Now $sin(frac{1}{2x}+O(1/x^3))= frac{1}{2x}+O(1/x^2)$ as $xrightarrow infty$, thus the expression is $frac{1}{2}+ O(frac{1}{x})$.
$endgroup$
add a comment |
$begingroup$
The ordo calculus is very useful for such problems. Sometimes you do not need the full power of Taylor series, only the first couple of coefficients.
$sqrt{x^2+3}- sqrt{x^2+3} = x(sqrt{1+3/x^2}- sqrt{1+2/x^2})=$
$= x(1+frac{1}{2}cdot(3/x^2)+O(1/x^4)- 1-frac{1}{2}cdot(2/x^2)+O(1/x^4))= frac{1}{2x}+O(1/x^3)$.
Now $sin(frac{1}{2x}+O(1/x^3))= frac{1}{2x}+O(1/x^2)$ as $xrightarrow infty$, thus the expression is $frac{1}{2}+ O(frac{1}{x})$.
$endgroup$
The ordo calculus is very useful for such problems. Sometimes you do not need the full power of Taylor series, only the first couple of coefficients.
$sqrt{x^2+3}- sqrt{x^2+3} = x(sqrt{1+3/x^2}- sqrt{1+2/x^2})=$
$= x(1+frac{1}{2}cdot(3/x^2)+O(1/x^4)- 1-frac{1}{2}cdot(2/x^2)+O(1/x^4))= frac{1}{2x}+O(1/x^3)$.
Now $sin(frac{1}{2x}+O(1/x^3))= frac{1}{2x}+O(1/x^2)$ as $xrightarrow infty$, thus the expression is $frac{1}{2}+ O(frac{1}{x})$.
edited Dec 31 '18 at 13:52
answered Dec 31 '18 at 13:40
A. PongráczA. Pongrácz
5,9981929
5,9981929
add a comment |
add a comment |
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