I need a hand with this integral? probably not even that difficult?

[jeebs]

Hi,
I need to evaluate \int e\stackrel{-(r/R)}{}sin(qr/h)dr between r=0 and r=\infty but I keep getting stuck.

I hae tried integrating by parts but the integral just keeps getting more complicated, and I have tried substituting in sin(x) = (1/2i)(e^ix - e^-ix) but that gave me the result that the whole thing is equal to infinity, which I know to be wrong.

can anybody suggest some sort of method or substitution that I could use?
this is driving me up the wall now.
cheers.

 

[Donaldos]

and I have tried substituting in sin(x) = (1/2i)(e^ix - e^-ix) but that gave me the result that the whole thing is equal to infinity, which I know to be wrong.

And yet it should have worked... Did you find an antiderivative for e^{-\frac r R}\sin\left(\frac{qr}{h}\right) ?

 

[Mark44]

Your first approach would have worked if you chose the right things for u and dv, and if you had done integration by parts a second time.

For the first integration by parts, choose u = e^-Ar and dv = cos(Br)dr. For the sake of simplicity, A = 1/R and B = q/h.

That should give you an integral with e^-Ar and sin(Br). Perform integration by parts a second time, with u = e^-Ar and dv = sin(Br)dr. This second integration by parts will give you an integral with e^-Ar and cos(Br), which is the same as what you started with. At this point you will have your original integral on the left side, and some expression plus a multiple of your original integral on the right side.

Bring both integral terms to one side and solve algebraically for your integral.

 

[tiny-tim]

Hi jeebs!

(have an integral: ∫ and an infinity: ∞ and try using the X² tag just above the Reply box )

Hint: what is e^-r/Rsin(qr/h) the imaginary part of?

 

[HallsofIvy]

Let u=e^{r/R} and dv= sin(qr/h)dr. Then du= e^{r/R}/R dr and v= -(h/q) cos(qr/h).

\int e^{r/R}sin(qr/h)dr= -(h/q)e^{r/R}cos(qr/h)+ (h/(qR))\int e^{r/R}cos(qr/h)dr. Now do it again, again letting u= e^{r/R} and dv= sin(qr/h)r. You will get a term involving \int e^{r/R}sin(qr/h)dr again. Do not integrate by parts a third time. Instead, solve the equation, algebraically, for \int e^{r/R}sin(qr/h)dr.