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Can't do this integral

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Original post by Khallil
I honestly haven't got the slightest clue. I just recognised the form from another question I'd seen. :facepalm2:



Do you mean co-ordinates or ordinates? They are two different (but somewhat linked) terms.

At first glance, it seems like you need to rearrange your first equation for tt in terms of xx, substitute it into your second equation so that you have yy in terms of xx and finally use your point to find the value of aa.


Co-ordinates, sorry.
Original post by n00bfi
Co-ordinates, sorry.


Where do you get all these fiendish questions from?

I had a go and made some progress by taking the 2nd equation and writing it in exponential form. By going down that route you can solve the 2nd equation for t and make progress from there.
Original post by QuantumOverlord
Where do you get all these fiendish questions from?

I had a go and made some progress by taking the 2nd equation and writing it in exponential form. By going down that route you can solve the 2nd equation for t and make progress from there.


My physics teacher and the internet :biggrin:

what do you mean, writing it in exponential form?
Original post by n00bfi
My physics teacher and the internet :biggrin:

what do you mean, writing it in exponential form?


I don't know if this is right, but my approach was to take this equation

t^2 * sint = pi^2 / 4 and write sine in exponential format

i.e change this equation to

i/2*t^2(exp(it) - exp(-it)) = pi^2 / 4

and go from there. I mean t has to be real, so thats a useful property I would be looking to exploit.
Original post by QuantumOverlord
I don't know if this is right, but my approach was to take this equation

t^2 * sint = pi^2 / 4 and write sine in exponential format

i.e change this equation to

i/2*t^2(exp(it) - exp(-it)) = pi^2 / 4

and go from there. I mean t has to be real, so thats a useful property I would be looking to exploit.


By inspection I saw t=±π2t = \pm \frac {\pi}{2} but my question lies in how to solve an equation of form x2sinx=k2 x^2sinx = k^2
(edited 10 years ago)
Guys halp me again pls pls

sinxsinx1dx\, \int \dfrac {sinx}{sinx-1} dx
Reply 46
Original post by n00bfi
Guys halp me again pls pls

sinxsinx1dx\, \int \dfrac {sinx}{sinx-1} dx


It looks like this should succumb to the x=tant2x=\tan\frac{t}{2} substitution followed by some partial fraction work.
Reply 47
Original post by n00bfi
Guys halp me again pls pls

sinxsinx1dx\, \int \dfrac {sinx}{sinx-1} dx


I haven't tried this, but if you multiply top and bottom by (sin x + 1) do you get something that's easier to work with?
Original post by n00bfi
sinxsinx1 dx\displaystyle \int \dfrac {\sin x}{\sin x-1} \text{ d}x


Original post by atsruser
x=tant2x=\tan\frac{t}{2}


t=tan(x2)t= \tan \left( \frac{x}{2} \right)

(edited 10 years ago)
Reply 49
Original post by Khallil
t=tan(x2)t= \tan \left( \frac{x}{2} \right)


Well-spotted.
Original post by atsruser
Well-spotted.


Thanks :smile:

I just finished the integral with that sub and it yielded a pretty cool result.

sinxsinx1 dx=t=tanx2 x21tanx2+C   x2cosx2cosx2sinx2+C\displaystyle \begin{aligned} \int \dfrac{\sin x}{\sin x - 1} \text{ d}x & \overset{t=\tan \frac{x}{2}}= \ x - \dfrac{2}{1 - \tan \frac{x}{2}} + \mathcal{C} \\ & \ \ \equiv \ x - \dfrac{2\cos \frac{x}{2}}{\cos \frac{x}{2} - \sin \frac{x}{2}} + \mathcal{C} \end{aligned}
(edited 10 years ago)
Original post by Khallil
Thanks :smile:

I just finished the integral with that sub and it yielded a pretty cool result.

sinxsinx1 dx=t=tanx2 x21tanx2+C   x2cosx2cosx2sinx2+C\displaystyle \begin{aligned} \int \dfrac{\sin x}{\sin x - 1} \text{ d}x & \overset{t=\tan \frac{x}{2}}= \ x - \dfrac{2}{1 - \tan \frac{x}{2}} + \mathcal{C} \\ & \ \ \equiv \ x - \dfrac{2\cos \frac{x}{2}}{\cos \frac{x}{2} - \sin \frac{x}{2}} + \mathcal{C} \end{aligned}


The world's sneakiest substitution! :colone:

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Original post by majmuh24
The world's sneakiest substitution! :colone:


Isn't it just :sexface:

Also, I had a little read on integral transforms. Laplace is such a boss!

L[f(t)](s)=0f(t)ets dt\mathcal{L} \left[ f(t) \right](s) = \displaystyle \int_{0}^{\infty} f(t) e^{-ts} \text{ d}t
(edited 10 years ago)
Original post by Khallil
Isn't it just :sexface:Also, I had a little read about integral transforms. Laplace is such a boss!L[f(t)](s)=0f(t)ets dt\mathcal{L} \left[ f(t) \right](s) = \displaystyle \int_{0}^{\infty} f(t) e^{-ts} \text{ d}t
All about Fourier man :wink:
Original post by majmuh24
All about Fourier man :wink:


:hand: Haha, Fourier is good too, but not a Leibniz. Have you seen his biscuits? :teehee:

(edited 10 years ago)
Back with one more:

11+xdx\int \dfrac {1}{1+\sqrt{x}} dx

PS how do I the big integral sign?
Original post by n00bfi
Back with one more:

11+xdx\int \dfrac {1}{1+\sqrt{x}} dx

PS how do I the big integral sign?


u=xu=\sqrt{x}

To do a big integral sign you need to add "\displaystyle" before the "\int" command.
Original post by Tarquin Digby
u=xu=\sqrt{x}

To do a big integral sign you need to add "\displaystyle" before the "\int" command.


Just did it lol, the exact same substitution. Well, I used x=u2x=u^2 but that's the same thing lol.

And thanks!

Moar:

sinx(π6)dx\displaystyle \int sin^x(\dfrac{\pi}{6}) dx

Well that basically comes out to be

2xdx\displaystyle \int 2^-x dx
(edited 10 years ago)
Original post by n00bfi
Just did it lol, the exact same substitution. Well, I used x=u2x=u^2 but that's the same thing lol.

And thanks!

Moar:

sinx(π6)\displaystyle \int sin^x(\dfrac{\pi}{6})


sinx(π6)dx=2xdx=2xlog2+C\displaystyle\int \sin^x\left(\dfrac{\pi}{6}\right)\, \mathrm{d} x=\int 2^{-x}\, \mathrm{d} x = -\frac{2^{-x}}{\log{2}}+C

Where are you getting these from?
Original post by Tarquin Digby
sinx(π6)dx=2xdx=2xlog2+C\displaystyle\int \sin^x\left(\dfrac{\pi}{6}\right)\, \mathrm{d} x=\int 2^{-x}\, \mathrm{d} x = -\frac{2^{-x}}{\log{2}}+C

Where are you getting these from?


From here: http://www.physicsforums.com/showpost.php?p=3433157&

What do you think of them?b

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