solving $limlimits_{xrightarrowinfty} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}$
$begingroup$
To investigate the convergence of a series I have to solve the folliwing limit:
begin{equation}
limlimits_{xrightarrowinfty} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
end{equation}
It should be $frac{1}{2}$ but i can't quite seem to get to that solution. I've tried to factor the square root out of the quotient which was:
begin{equation}
limlimits_{xrightarrowinfty} sqrt{frac{((x^2-1) sqrt{x + 2}-x^2sqrt{x+1})^2}{x^2(x + 1)}}
end{equation}
Then i worked out the square in the numerator which was:
begin{equation}
(x^2-1)^2 (x + 2)-2x^2(x^2-1)sqrt{x+1}sqrt{x+2}+x^4(x+1)
end{equation}
I could then factor the terms and take and divide the numerator with the (x+1) from the denominator. Then i could expand the terms in the numerator wich became:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{(x^4+x^3-3x^2-x+2)-sqrt{16x^8+32x^7-12x^6-20x^5+8x^4}+x^4}{x^2}}
end{equation}
Now i can take the $16x^8$ out of the root and then i then looked at the terms with the highest exponent so i had:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{x^4-4x^4+x^4}{x^2}} =limlimits_{xrightarrowinfty}sqrt{frac{-2x^2}{x^2}}
end{equation}
Which could only be solved with complex numbers, so i should be wrong somewhere in my calculations, since i know that i should get $frac{1}{2}$. I also checked my sollution with WolframAlpha which also gave $frac{1}{2}$ so I know that the sollution i have is correct.
Would anyone know where i was wrong or how i could better solve it?
real-analysis calculus convergence
$endgroup$
add a comment |
$begingroup$
To investigate the convergence of a series I have to solve the folliwing limit:
begin{equation}
limlimits_{xrightarrowinfty} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
end{equation}
It should be $frac{1}{2}$ but i can't quite seem to get to that solution. I've tried to factor the square root out of the quotient which was:
begin{equation}
limlimits_{xrightarrowinfty} sqrt{frac{((x^2-1) sqrt{x + 2}-x^2sqrt{x+1})^2}{x^2(x + 1)}}
end{equation}
Then i worked out the square in the numerator which was:
begin{equation}
(x^2-1)^2 (x + 2)-2x^2(x^2-1)sqrt{x+1}sqrt{x+2}+x^4(x+1)
end{equation}
I could then factor the terms and take and divide the numerator with the (x+1) from the denominator. Then i could expand the terms in the numerator wich became:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{(x^4+x^3-3x^2-x+2)-sqrt{16x^8+32x^7-12x^6-20x^5+8x^4}+x^4}{x^2}}
end{equation}
Now i can take the $16x^8$ out of the root and then i then looked at the terms with the highest exponent so i had:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{x^4-4x^4+x^4}{x^2}} =limlimits_{xrightarrowinfty}sqrt{frac{-2x^2}{x^2}}
end{equation}
Which could only be solved with complex numbers, so i should be wrong somewhere in my calculations, since i know that i should get $frac{1}{2}$. I also checked my sollution with WolframAlpha which also gave $frac{1}{2}$ so I know that the sollution i have is correct.
Would anyone know where i was wrong or how i could better solve it?
real-analysis calculus convergence
$endgroup$
add a comment |
$begingroup$
To investigate the convergence of a series I have to solve the folliwing limit:
begin{equation}
limlimits_{xrightarrowinfty} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
end{equation}
It should be $frac{1}{2}$ but i can't quite seem to get to that solution. I've tried to factor the square root out of the quotient which was:
begin{equation}
limlimits_{xrightarrowinfty} sqrt{frac{((x^2-1) sqrt{x + 2}-x^2sqrt{x+1})^2}{x^2(x + 1)}}
end{equation}
Then i worked out the square in the numerator which was:
begin{equation}
(x^2-1)^2 (x + 2)-2x^2(x^2-1)sqrt{x+1}sqrt{x+2}+x^4(x+1)
end{equation}
I could then factor the terms and take and divide the numerator with the (x+1) from the denominator. Then i could expand the terms in the numerator wich became:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{(x^4+x^3-3x^2-x+2)-sqrt{16x^8+32x^7-12x^6-20x^5+8x^4}+x^4}{x^2}}
end{equation}
Now i can take the $16x^8$ out of the root and then i then looked at the terms with the highest exponent so i had:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{x^4-4x^4+x^4}{x^2}} =limlimits_{xrightarrowinfty}sqrt{frac{-2x^2}{x^2}}
end{equation}
Which could only be solved with complex numbers, so i should be wrong somewhere in my calculations, since i know that i should get $frac{1}{2}$. I also checked my sollution with WolframAlpha which also gave $frac{1}{2}$ so I know that the sollution i have is correct.
Would anyone know where i was wrong or how i could better solve it?
real-analysis calculus convergence
$endgroup$
To investigate the convergence of a series I have to solve the folliwing limit:
begin{equation}
limlimits_{xrightarrowinfty} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
end{equation}
It should be $frac{1}{2}$ but i can't quite seem to get to that solution. I've tried to factor the square root out of the quotient which was:
begin{equation}
limlimits_{xrightarrowinfty} sqrt{frac{((x^2-1) sqrt{x + 2}-x^2sqrt{x+1})^2}{x^2(x + 1)}}
end{equation}
Then i worked out the square in the numerator which was:
begin{equation}
(x^2-1)^2 (x + 2)-2x^2(x^2-1)sqrt{x+1}sqrt{x+2}+x^4(x+1)
end{equation}
I could then factor the terms and take and divide the numerator with the (x+1) from the denominator. Then i could expand the terms in the numerator wich became:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{(x^4+x^3-3x^2-x+2)-sqrt{16x^8+32x^7-12x^6-20x^5+8x^4}+x^4}{x^2}}
end{equation}
Now i can take the $16x^8$ out of the root and then i then looked at the terms with the highest exponent so i had:
begin{equation}
limlimits_{xrightarrowinfty}sqrt{frac{x^4-4x^4+x^4}{x^2}} =limlimits_{xrightarrowinfty}sqrt{frac{-2x^2}{x^2}}
end{equation}
Which could only be solved with complex numbers, so i should be wrong somewhere in my calculations, since i know that i should get $frac{1}{2}$. I also checked my sollution with WolframAlpha which also gave $frac{1}{2}$ so I know that the sollution i have is correct.
Would anyone know where i was wrong or how i could better solve it?
real-analysis calculus convergence
real-analysis calculus convergence
asked Jan 7 at 15:49
ViktorViktor
1389
1389
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2 Answers
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$begingroup$
$$lim_{xtoinfty}frac{(x^2-1)sqrt{x+2}-x^2sqrt{x+1}}{xsqrt{x+1}}=lim_{xtoinfty}frac{x^2big[sqrt{x+2}-sqrt{x+1}big]}{xsqrt{x+1}}-frac{sqrt{x+2}}{xsqrt{x+1}}$$
The latter goes to $0$ since $$lim_{xtoinfty}frac{sqrt{x+2}}{xsqrt{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{frac{x+2}{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{1+frac1{x+1}}=0$$
You are left with $$lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}=lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}cdotfrac{big[sqrt{x+2}+sqrt{x+1}big]}{big[sqrt{x+2}+sqrt{x+1}big]}\=lim_{xtoinfty}frac x{sqrt{x+1}big[sqrt{x+2}+sqrt{x+1}big]}=lim_{xtoinfty}frac 1{sqrt{1+frac1x}Big[sqrt{1+frac2x}+sqrt{1+frac1x}Big]}=1/2$$
$endgroup$
add a comment |
$begingroup$
Another way:
begin{eqnarray*} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
& = & frac{(x-1)(x+1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}} \
& = & frac{(x-1)sqrt{(x+1)(x + 2)}-x^2}{x} \
& stackrel{x^2 = x(x-1)+x}{=} & underbrace{frac{x-1}{x}}_{stackrel{x to +infty}{longrightarrow}1}underbrace{left(sqrt{(x+1)(x + 2)} - xright)}_{= frac{3x+2}{sqrt{(x+1)(x + 2)} + x}stackrel{x to +infty}{longrightarrow}frac{3}{2}} - 1 \
& stackrel{x to +infty}{longrightarrow} & 1 cdot frac{3}{2} -1 = frac{1}{2}
end{eqnarray*}
$endgroup$
add a comment |
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2 Answers
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2 Answers
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active
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$begingroup$
$$lim_{xtoinfty}frac{(x^2-1)sqrt{x+2}-x^2sqrt{x+1}}{xsqrt{x+1}}=lim_{xtoinfty}frac{x^2big[sqrt{x+2}-sqrt{x+1}big]}{xsqrt{x+1}}-frac{sqrt{x+2}}{xsqrt{x+1}}$$
The latter goes to $0$ since $$lim_{xtoinfty}frac{sqrt{x+2}}{xsqrt{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{frac{x+2}{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{1+frac1{x+1}}=0$$
You are left with $$lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}=lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}cdotfrac{big[sqrt{x+2}+sqrt{x+1}big]}{big[sqrt{x+2}+sqrt{x+1}big]}\=lim_{xtoinfty}frac x{sqrt{x+1}big[sqrt{x+2}+sqrt{x+1}big]}=lim_{xtoinfty}frac 1{sqrt{1+frac1x}Big[sqrt{1+frac2x}+sqrt{1+frac1x}Big]}=1/2$$
$endgroup$
add a comment |
$begingroup$
$$lim_{xtoinfty}frac{(x^2-1)sqrt{x+2}-x^2sqrt{x+1}}{xsqrt{x+1}}=lim_{xtoinfty}frac{x^2big[sqrt{x+2}-sqrt{x+1}big]}{xsqrt{x+1}}-frac{sqrt{x+2}}{xsqrt{x+1}}$$
The latter goes to $0$ since $$lim_{xtoinfty}frac{sqrt{x+2}}{xsqrt{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{frac{x+2}{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{1+frac1{x+1}}=0$$
You are left with $$lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}=lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}cdotfrac{big[sqrt{x+2}+sqrt{x+1}big]}{big[sqrt{x+2}+sqrt{x+1}big]}\=lim_{xtoinfty}frac x{sqrt{x+1}big[sqrt{x+2}+sqrt{x+1}big]}=lim_{xtoinfty}frac 1{sqrt{1+frac1x}Big[sqrt{1+frac2x}+sqrt{1+frac1x}Big]}=1/2$$
$endgroup$
add a comment |
$begingroup$
$$lim_{xtoinfty}frac{(x^2-1)sqrt{x+2}-x^2sqrt{x+1}}{xsqrt{x+1}}=lim_{xtoinfty}frac{x^2big[sqrt{x+2}-sqrt{x+1}big]}{xsqrt{x+1}}-frac{sqrt{x+2}}{xsqrt{x+1}}$$
The latter goes to $0$ since $$lim_{xtoinfty}frac{sqrt{x+2}}{xsqrt{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{frac{x+2}{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{1+frac1{x+1}}=0$$
You are left with $$lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}=lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}cdotfrac{big[sqrt{x+2}+sqrt{x+1}big]}{big[sqrt{x+2}+sqrt{x+1}big]}\=lim_{xtoinfty}frac x{sqrt{x+1}big[sqrt{x+2}+sqrt{x+1}big]}=lim_{xtoinfty}frac 1{sqrt{1+frac1x}Big[sqrt{1+frac2x}+sqrt{1+frac1x}Big]}=1/2$$
$endgroup$
$$lim_{xtoinfty}frac{(x^2-1)sqrt{x+2}-x^2sqrt{x+1}}{xsqrt{x+1}}=lim_{xtoinfty}frac{x^2big[sqrt{x+2}-sqrt{x+1}big]}{xsqrt{x+1}}-frac{sqrt{x+2}}{xsqrt{x+1}}$$
The latter goes to $0$ since $$lim_{xtoinfty}frac{sqrt{x+2}}{xsqrt{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{frac{x+2}{x+1}}=lim_{xtoinfty}frac1xcdotsqrt{1+frac1{x+1}}=0$$
You are left with $$lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}=lim_{xtoinfty}frac{xbig[sqrt{x+2}-sqrt{x+1}big]}{sqrt{x+1}}cdotfrac{big[sqrt{x+2}+sqrt{x+1}big]}{big[sqrt{x+2}+sqrt{x+1}big]}\=lim_{xtoinfty}frac x{sqrt{x+1}big[sqrt{x+2}+sqrt{x+1}big]}=lim_{xtoinfty}frac 1{sqrt{1+frac1x}Big[sqrt{1+frac2x}+sqrt{1+frac1x}Big]}=1/2$$
answered Jan 7 at 16:13


Shubham JohriShubham Johri
5,017717
5,017717
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$begingroup$
Another way:
begin{eqnarray*} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
& = & frac{(x-1)(x+1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}} \
& = & frac{(x-1)sqrt{(x+1)(x + 2)}-x^2}{x} \
& stackrel{x^2 = x(x-1)+x}{=} & underbrace{frac{x-1}{x}}_{stackrel{x to +infty}{longrightarrow}1}underbrace{left(sqrt{(x+1)(x + 2)} - xright)}_{= frac{3x+2}{sqrt{(x+1)(x + 2)} + x}stackrel{x to +infty}{longrightarrow}frac{3}{2}} - 1 \
& stackrel{x to +infty}{longrightarrow} & 1 cdot frac{3}{2} -1 = frac{1}{2}
end{eqnarray*}
$endgroup$
add a comment |
$begingroup$
Another way:
begin{eqnarray*} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
& = & frac{(x-1)(x+1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}} \
& = & frac{(x-1)sqrt{(x+1)(x + 2)}-x^2}{x} \
& stackrel{x^2 = x(x-1)+x}{=} & underbrace{frac{x-1}{x}}_{stackrel{x to +infty}{longrightarrow}1}underbrace{left(sqrt{(x+1)(x + 2)} - xright)}_{= frac{3x+2}{sqrt{(x+1)(x + 2)} + x}stackrel{x to +infty}{longrightarrow}frac{3}{2}} - 1 \
& stackrel{x to +infty}{longrightarrow} & 1 cdot frac{3}{2} -1 = frac{1}{2}
end{eqnarray*}
$endgroup$
add a comment |
$begingroup$
Another way:
begin{eqnarray*} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
& = & frac{(x-1)(x+1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}} \
& = & frac{(x-1)sqrt{(x+1)(x + 2)}-x^2}{x} \
& stackrel{x^2 = x(x-1)+x}{=} & underbrace{frac{x-1}{x}}_{stackrel{x to +infty}{longrightarrow}1}underbrace{left(sqrt{(x+1)(x + 2)} - xright)}_{= frac{3x+2}{sqrt{(x+1)(x + 2)} + x}stackrel{x to +infty}{longrightarrow}frac{3}{2}} - 1 \
& stackrel{x to +infty}{longrightarrow} & 1 cdot frac{3}{2} -1 = frac{1}{2}
end{eqnarray*}
$endgroup$
Another way:
begin{eqnarray*} frac{(x^2-1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}}
& = & frac{(x-1)(x+1) sqrt{x + 2}-x^2sqrt{x+1}}{xsqrt{x + 1}} \
& = & frac{(x-1)sqrt{(x+1)(x + 2)}-x^2}{x} \
& stackrel{x^2 = x(x-1)+x}{=} & underbrace{frac{x-1}{x}}_{stackrel{x to +infty}{longrightarrow}1}underbrace{left(sqrt{(x+1)(x + 2)} - xright)}_{= frac{3x+2}{sqrt{(x+1)(x + 2)} + x}stackrel{x to +infty}{longrightarrow}frac{3}{2}} - 1 \
& stackrel{x to +infty}{longrightarrow} & 1 cdot frac{3}{2} -1 = frac{1}{2}
end{eqnarray*}
answered Jan 7 at 17:07
trancelocationtrancelocation
10.6k1722
10.6k1722
add a comment |
add a comment |
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