How do I find the length of the polar spiral $r = acdot theta^2 - 1 + bcdot theta$?












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How do I find the length of this polar spiral?



$$r = acdot theta^2 - 1 + bcdot theta,$$ where $a$ and $b$ are variables.











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  • 2




    $begingroup$
    Do you know a formula for arc length using polar coordinates?
    $endgroup$
    – Zubin Mukerjee
    Jan 21 at 12:07






  • 2




    $begingroup$
    khanacademy.org/math/integral-calculus/ic-adv-funcs/…
    $endgroup$
    – Mohammad Zuhair Khan
    Jan 21 at 12:11
















-1












$begingroup$



How do I find the length of this polar spiral?



$$r = acdot theta^2 - 1 + bcdot theta,$$ where $a$ and $b$ are variables.











share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Do you know a formula for arc length using polar coordinates?
    $endgroup$
    – Zubin Mukerjee
    Jan 21 at 12:07






  • 2




    $begingroup$
    khanacademy.org/math/integral-calculus/ic-adv-funcs/…
    $endgroup$
    – Mohammad Zuhair Khan
    Jan 21 at 12:11














-1












-1








-1





$begingroup$



How do I find the length of this polar spiral?



$$r = acdot theta^2 - 1 + bcdot theta,$$ where $a$ and $b$ are variables.











share|cite|improve this question











$endgroup$





How do I find the length of this polar spiral?



$$r = acdot theta^2 - 1 + bcdot theta,$$ where $a$ and $b$ are variables.








graphing-functions






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share|cite|improve this question













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share|cite|improve this question








edited Jan 21 at 13:00









Mohammad Zuhair Khan

1,6662625




1,6662625










asked Jan 21 at 12:04









user33915user33915

11




11








  • 2




    $begingroup$
    Do you know a formula for arc length using polar coordinates?
    $endgroup$
    – Zubin Mukerjee
    Jan 21 at 12:07






  • 2




    $begingroup$
    khanacademy.org/math/integral-calculus/ic-adv-funcs/…
    $endgroup$
    – Mohammad Zuhair Khan
    Jan 21 at 12:11














  • 2




    $begingroup$
    Do you know a formula for arc length using polar coordinates?
    $endgroup$
    – Zubin Mukerjee
    Jan 21 at 12:07






  • 2




    $begingroup$
    khanacademy.org/math/integral-calculus/ic-adv-funcs/…
    $endgroup$
    – Mohammad Zuhair Khan
    Jan 21 at 12:11








2




2




$begingroup$
Do you know a formula for arc length using polar coordinates?
$endgroup$
– Zubin Mukerjee
Jan 21 at 12:07




$begingroup$
Do you know a formula for arc length using polar coordinates?
$endgroup$
– Zubin Mukerjee
Jan 21 at 12:07




2




2




$begingroup$
khanacademy.org/math/integral-calculus/ic-adv-funcs/…
$endgroup$
– Mohammad Zuhair Khan
Jan 21 at 12:11




$begingroup$
khanacademy.org/math/integral-calculus/ic-adv-funcs/…
$endgroup$
– Mohammad Zuhair Khan
Jan 21 at 12:11










1 Answer
1






active

oldest

votes


















1












$begingroup$

The formula tells us ( assuming $;0lethetale2pi;$ )



$$mathcal L=int_0^{2pi}sqrt{r^2+r_theta^2} ,dtheta=int_0^{2pi}sqrt{(atheta^2+btheta-1)^2+(2atheta+b)^2},dtheta$$



Not the nicest integral to see...but give it a try.



Pay attention to the fact that



$$r_theta^2=left(frac{dr}{dtheta}right)^2$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
    $endgroup$
    – DonAntonio
    Jan 23 at 12:50










  • $begingroup$
    @user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
    $endgroup$
    – DonAntonio
    Jan 23 at 13:53













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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1












$begingroup$

The formula tells us ( assuming $;0lethetale2pi;$ )



$$mathcal L=int_0^{2pi}sqrt{r^2+r_theta^2} ,dtheta=int_0^{2pi}sqrt{(atheta^2+btheta-1)^2+(2atheta+b)^2},dtheta$$



Not the nicest integral to see...but give it a try.



Pay attention to the fact that



$$r_theta^2=left(frac{dr}{dtheta}right)^2$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
    $endgroup$
    – DonAntonio
    Jan 23 at 12:50










  • $begingroup$
    @user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
    $endgroup$
    – DonAntonio
    Jan 23 at 13:53


















1












$begingroup$

The formula tells us ( assuming $;0lethetale2pi;$ )



$$mathcal L=int_0^{2pi}sqrt{r^2+r_theta^2} ,dtheta=int_0^{2pi}sqrt{(atheta^2+btheta-1)^2+(2atheta+b)^2},dtheta$$



Not the nicest integral to see...but give it a try.



Pay attention to the fact that



$$r_theta^2=left(frac{dr}{dtheta}right)^2$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
    $endgroup$
    – DonAntonio
    Jan 23 at 12:50










  • $begingroup$
    @user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
    $endgroup$
    – DonAntonio
    Jan 23 at 13:53
















1












1








1





$begingroup$

The formula tells us ( assuming $;0lethetale2pi;$ )



$$mathcal L=int_0^{2pi}sqrt{r^2+r_theta^2} ,dtheta=int_0^{2pi}sqrt{(atheta^2+btheta-1)^2+(2atheta+b)^2},dtheta$$



Not the nicest integral to see...but give it a try.



Pay attention to the fact that



$$r_theta^2=left(frac{dr}{dtheta}right)^2$$






share|cite|improve this answer









$endgroup$



The formula tells us ( assuming $;0lethetale2pi;$ )



$$mathcal L=int_0^{2pi}sqrt{r^2+r_theta^2} ,dtheta=int_0^{2pi}sqrt{(atheta^2+btheta-1)^2+(2atheta+b)^2},dtheta$$



Not the nicest integral to see...but give it a try.



Pay attention to the fact that



$$r_theta^2=left(frac{dr}{dtheta}right)^2$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 21 at 12:11









DonAntonioDonAntonio

179k1494232




179k1494232












  • $begingroup$
    I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
    $endgroup$
    – DonAntonio
    Jan 23 at 12:50










  • $begingroup$
    @user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
    $endgroup$
    – DonAntonio
    Jan 23 at 13:53




















  • $begingroup$
    I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
    $endgroup$
    – DonAntonio
    Jan 23 at 12:50










  • $begingroup$
    @user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
    $endgroup$
    – DonAntonio
    Jan 23 at 13:53


















$begingroup$
I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
$endgroup$
– DonAntonio
Jan 23 at 12:50




$begingroup$
I've no idea. I haven't even tried to solve that integral: it looks nasty. Yet you can try to do it with Wolfram or some other program...
$endgroup$
– DonAntonio
Jan 23 at 12:50












$begingroup$
@user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
$endgroup$
– DonAntonio
Jan 23 at 13:53






$begingroup$
@user33915 I'm sorry but I still have no idea, yet this time it is because I can't understand what you mean: why the integral changes and all?? And how come you want to integrate with the variable ($,theta,$) being fixed?
$endgroup$
– DonAntonio
Jan 23 at 13:53




















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