Integral of $frac{x^2}{sqrt{x^2+5}}$
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I need help with this integral:
$$I = int frac{x^2}{sqrt{x^2+5}}, dx$$
I substituted $x = sqrt{5}tan{theta}$, and reached $$I = 5int frac{sin{(theta)}^2}{cos{(theta)}^3},dtheta$$ which I'm unable to solve.
For context this is in an exercise of trig substitution, and I'm not allowed to use the secant reduction formula which WolframAlpha suggests I use.
calculus integration trigonometry
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add a comment |
$begingroup$
I need help with this integral:
$$I = int frac{x^2}{sqrt{x^2+5}}, dx$$
I substituted $x = sqrt{5}tan{theta}$, and reached $$I = 5int frac{sin{(theta)}^2}{cos{(theta)}^3},dtheta$$ which I'm unable to solve.
For context this is in an exercise of trig substitution, and I'm not allowed to use the secant reduction formula which WolframAlpha suggests I use.
calculus integration trigonometry
$endgroup$
add a comment |
$begingroup$
I need help with this integral:
$$I = int frac{x^2}{sqrt{x^2+5}}, dx$$
I substituted $x = sqrt{5}tan{theta}$, and reached $$I = 5int frac{sin{(theta)}^2}{cos{(theta)}^3},dtheta$$ which I'm unable to solve.
For context this is in an exercise of trig substitution, and I'm not allowed to use the secant reduction formula which WolframAlpha suggests I use.
calculus integration trigonometry
$endgroup$
I need help with this integral:
$$I = int frac{x^2}{sqrt{x^2+5}}, dx$$
I substituted $x = sqrt{5}tan{theta}$, and reached $$I = 5int frac{sin{(theta)}^2}{cos{(theta)}^3},dtheta$$ which I'm unable to solve.
For context this is in an exercise of trig substitution, and I'm not allowed to use the secant reduction formula which WolframAlpha suggests I use.
calculus integration trigonometry
calculus integration trigonometry
edited Jan 18 at 13:28
Robert Z
102k1072145
102k1072145
asked Jan 18 at 12:55
Yizhar AmirYizhar Amir
17217
17217
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add a comment |
6 Answers
6
active
oldest
votes
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Hint: (Use hyperbolic trigonometric substitution to)
$$
frac{x^2}{sqrt{x^2+5}}=sqrt{x^2+5}-frac{5}{sqrt{x^2+5}}.
$$
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add a comment |
$begingroup$
Note that$$frac{sin^2(theta)}{cos^3(theta)}=frac{sin^2(theta)cos(theta)}{bigl(1-sin^2(theta)bigr)^2}.$$So, you can do $sin(theta)=u$ and $cos(theta),mathrm dtheta=mathrm du$.
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This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
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– GEdgar
Jan 18 at 13:08
add a comment |
$begingroup$
Hint. Let $t$ such that $x^2+5=(x-t)^2$, that is $x=(t^2-5)/(2t)$. Then new integral is the integral of a rational function (very easy to calculate):
$$int frac{x^2}{sqrt{x^2+5}}, dx=int left(frac{25}{4t^3}+frac{t}{4}-frac{5}{2t}right), dt.$$
Finally note that $t=x+sqrt{x^2+5}$.
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add a comment |
$begingroup$
Hint: Rewrite $intfrac{sin^2{(theta)}}{cos^3{(theta)}} d theta$ as $int tan^2 theta sec theta d theta$. Let $u = sec theta$; then $du = ?$ - this will reduce the integral to something trivial.
$endgroup$
add a comment |
$begingroup$
Hint: Substitute $y=sqrt{x^2+5}$. Then $dy= frac{x}{sqrt{x^2+5}}dx$, so the integral is $int sqrt{y^2-5} dy$.
$endgroup$
add a comment |
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What about
$$1+frac x{sqrt5}=sinh uimplies dx=cosh usqrt 5,duimplies$$
$$intfrac{x^2}{sqrt{x^2+5}}==frac1{sqrt5}intfrac{x^2}{sqrt{1+left(frac x{sqrt 5}right)^2}}dx=frac1{sqrt5}intfrac{(sqrt5,sinh u-1)^2}{cosh u}sqrt5,cosh u,du=$$
$$intleft(5sinh^2u-10sinh u+1right)du$$
and now it is almost an immediate integral.
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add a comment |
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6 Answers
6
active
oldest
votes
6 Answers
6
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint: (Use hyperbolic trigonometric substitution to)
$$
frac{x^2}{sqrt{x^2+5}}=sqrt{x^2+5}-frac{5}{sqrt{x^2+5}}.
$$
$endgroup$
add a comment |
$begingroup$
Hint: (Use hyperbolic trigonometric substitution to)
$$
frac{x^2}{sqrt{x^2+5}}=sqrt{x^2+5}-frac{5}{sqrt{x^2+5}}.
$$
$endgroup$
add a comment |
$begingroup$
Hint: (Use hyperbolic trigonometric substitution to)
$$
frac{x^2}{sqrt{x^2+5}}=sqrt{x^2+5}-frac{5}{sqrt{x^2+5}}.
$$
$endgroup$
Hint: (Use hyperbolic trigonometric substitution to)
$$
frac{x^2}{sqrt{x^2+5}}=sqrt{x^2+5}-frac{5}{sqrt{x^2+5}}.
$$
answered Jan 18 at 12:59
SongSong
18.6k21651
18.6k21651
add a comment |
add a comment |
$begingroup$
Note that$$frac{sin^2(theta)}{cos^3(theta)}=frac{sin^2(theta)cos(theta)}{bigl(1-sin^2(theta)bigr)^2}.$$So, you can do $sin(theta)=u$ and $cos(theta),mathrm dtheta=mathrm du$.
$endgroup$
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
add a comment |
$begingroup$
Note that$$frac{sin^2(theta)}{cos^3(theta)}=frac{sin^2(theta)cos(theta)}{bigl(1-sin^2(theta)bigr)^2}.$$So, you can do $sin(theta)=u$ and $cos(theta),mathrm dtheta=mathrm du$.
$endgroup$
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
add a comment |
$begingroup$
Note that$$frac{sin^2(theta)}{cos^3(theta)}=frac{sin^2(theta)cos(theta)}{bigl(1-sin^2(theta)bigr)^2}.$$So, you can do $sin(theta)=u$ and $cos(theta),mathrm dtheta=mathrm du$.
$endgroup$
Note that$$frac{sin^2(theta)}{cos^3(theta)}=frac{sin^2(theta)cos(theta)}{bigl(1-sin^2(theta)bigr)^2}.$$So, you can do $sin(theta)=u$ and $cos(theta),mathrm dtheta=mathrm du$.
answered Jan 18 at 13:03
José Carlos SantosJosé Carlos Santos
176k24135244
176k24135244
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
add a comment |
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
$begingroup$
This method works when you have power of $sin$ times power of $cos$ where at least one of the exponents is odd.
$endgroup$
– GEdgar
Jan 18 at 13:08
add a comment |
$begingroup$
Hint. Let $t$ such that $x^2+5=(x-t)^2$, that is $x=(t^2-5)/(2t)$. Then new integral is the integral of a rational function (very easy to calculate):
$$int frac{x^2}{sqrt{x^2+5}}, dx=int left(frac{25}{4t^3}+frac{t}{4}-frac{5}{2t}right), dt.$$
Finally note that $t=x+sqrt{x^2+5}$.
$endgroup$
add a comment |
$begingroup$
Hint. Let $t$ such that $x^2+5=(x-t)^2$, that is $x=(t^2-5)/(2t)$. Then new integral is the integral of a rational function (very easy to calculate):
$$int frac{x^2}{sqrt{x^2+5}}, dx=int left(frac{25}{4t^3}+frac{t}{4}-frac{5}{2t}right), dt.$$
Finally note that $t=x+sqrt{x^2+5}$.
$endgroup$
add a comment |
$begingroup$
Hint. Let $t$ such that $x^2+5=(x-t)^2$, that is $x=(t^2-5)/(2t)$. Then new integral is the integral of a rational function (very easy to calculate):
$$int frac{x^2}{sqrt{x^2+5}}, dx=int left(frac{25}{4t^3}+frac{t}{4}-frac{5}{2t}right), dt.$$
Finally note that $t=x+sqrt{x^2+5}$.
$endgroup$
Hint. Let $t$ such that $x^2+5=(x-t)^2$, that is $x=(t^2-5)/(2t)$. Then new integral is the integral of a rational function (very easy to calculate):
$$int frac{x^2}{sqrt{x^2+5}}, dx=int left(frac{25}{4t^3}+frac{t}{4}-frac{5}{2t}right), dt.$$
Finally note that $t=x+sqrt{x^2+5}$.
edited Jan 18 at 13:26
answered Jan 18 at 13:08
Robert ZRobert Z
102k1072145
102k1072145
add a comment |
add a comment |
$begingroup$
Hint: Rewrite $intfrac{sin^2{(theta)}}{cos^3{(theta)}} d theta$ as $int tan^2 theta sec theta d theta$. Let $u = sec theta$; then $du = ?$ - this will reduce the integral to something trivial.
$endgroup$
add a comment |
$begingroup$
Hint: Rewrite $intfrac{sin^2{(theta)}}{cos^3{(theta)}} d theta$ as $int tan^2 theta sec theta d theta$. Let $u = sec theta$; then $du = ?$ - this will reduce the integral to something trivial.
$endgroup$
add a comment |
$begingroup$
Hint: Rewrite $intfrac{sin^2{(theta)}}{cos^3{(theta)}} d theta$ as $int tan^2 theta sec theta d theta$. Let $u = sec theta$; then $du = ?$ - this will reduce the integral to something trivial.
$endgroup$
Hint: Rewrite $intfrac{sin^2{(theta)}}{cos^3{(theta)}} d theta$ as $int tan^2 theta sec theta d theta$. Let $u = sec theta$; then $du = ?$ - this will reduce the integral to something trivial.
answered Jan 18 at 13:43
bjcolby15bjcolby15
1,51711016
1,51711016
add a comment |
add a comment |
$begingroup$
Hint: Substitute $y=sqrt{x^2+5}$. Then $dy= frac{x}{sqrt{x^2+5}}dx$, so the integral is $int sqrt{y^2-5} dy$.
$endgroup$
add a comment |
$begingroup$
Hint: Substitute $y=sqrt{x^2+5}$. Then $dy= frac{x}{sqrt{x^2+5}}dx$, so the integral is $int sqrt{y^2-5} dy$.
$endgroup$
add a comment |
$begingroup$
Hint: Substitute $y=sqrt{x^2+5}$. Then $dy= frac{x}{sqrt{x^2+5}}dx$, so the integral is $int sqrt{y^2-5} dy$.
$endgroup$
Hint: Substitute $y=sqrt{x^2+5}$. Then $dy= frac{x}{sqrt{x^2+5}}dx$, so the integral is $int sqrt{y^2-5} dy$.
answered Jan 18 at 13:01
A. PongráczA. Pongrácz
6,1171929
6,1171929
add a comment |
add a comment |
$begingroup$
What about
$$1+frac x{sqrt5}=sinh uimplies dx=cosh usqrt 5,duimplies$$
$$intfrac{x^2}{sqrt{x^2+5}}==frac1{sqrt5}intfrac{x^2}{sqrt{1+left(frac x{sqrt 5}right)^2}}dx=frac1{sqrt5}intfrac{(sqrt5,sinh u-1)^2}{cosh u}sqrt5,cosh u,du=$$
$$intleft(5sinh^2u-10sinh u+1right)du$$
and now it is almost an immediate integral.
$endgroup$
add a comment |
$begingroup$
What about
$$1+frac x{sqrt5}=sinh uimplies dx=cosh usqrt 5,duimplies$$
$$intfrac{x^2}{sqrt{x^2+5}}==frac1{sqrt5}intfrac{x^2}{sqrt{1+left(frac x{sqrt 5}right)^2}}dx=frac1{sqrt5}intfrac{(sqrt5,sinh u-1)^2}{cosh u}sqrt5,cosh u,du=$$
$$intleft(5sinh^2u-10sinh u+1right)du$$
and now it is almost an immediate integral.
$endgroup$
add a comment |
$begingroup$
What about
$$1+frac x{sqrt5}=sinh uimplies dx=cosh usqrt 5,duimplies$$
$$intfrac{x^2}{sqrt{x^2+5}}==frac1{sqrt5}intfrac{x^2}{sqrt{1+left(frac x{sqrt 5}right)^2}}dx=frac1{sqrt5}intfrac{(sqrt5,sinh u-1)^2}{cosh u}sqrt5,cosh u,du=$$
$$intleft(5sinh^2u-10sinh u+1right)du$$
and now it is almost an immediate integral.
$endgroup$
What about
$$1+frac x{sqrt5}=sinh uimplies dx=cosh usqrt 5,duimplies$$
$$intfrac{x^2}{sqrt{x^2+5}}==frac1{sqrt5}intfrac{x^2}{sqrt{1+left(frac x{sqrt 5}right)^2}}dx=frac1{sqrt5}intfrac{(sqrt5,sinh u-1)^2}{cosh u}sqrt5,cosh u,du=$$
$$intleft(5sinh^2u-10sinh u+1right)du$$
and now it is almost an immediate integral.
answered Jan 18 at 13:03
DonAntonioDonAntonio
180k1495233
180k1495233
add a comment |
add a comment |
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