# Sarahisme

Senior Members

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1. ## Fourier Cosine series of cos(x)

its cool, got it! don't worry guys
2. ## Fourier Cosine series of cos(x)

Hello peoples, I think this is a trick question... well sort of for part (a) i get that the cosine Fourier Series for f(x) = cos(x) to be: i hope that is ok, but its part (b) that is troubling me... is all that happens as is that the cosine Fourier series of cos(x) goes to 0? i am guessing i am missing some trick to this question? Cheers! Sarah
3. ## Fourier series: Parseval's identity... HELP!

its cool everyone, got the question, its all good!
4. ## wave equation pde problem

ah ok, yeah that all makes sense now. since you people have been so helpful with this PDE question, if any of you guys have time, i would really love some help with some of my other questions thanks guys! Sarah
5. ## wave equation pde problem

but don't we also need to satify the initial conditions?
6. ## PDE problem

hmmm i have no idea where to even start with this problem, i cannot find any examples that are similar or anything like that anywhere! anyone got an idea as to a good first step to take? thanks sarah edit: i tryed something wild and came up with $A(u) = e^{u(x,y)}$ but i didnt use any knowlage of PDE's to work that out....hmmm.... :S
7. ## 2nd order PDE charcteristics... help please! :S

hi all, sorry again for the thousands of stupid questions! but heres another one... ok i said for part (a) that the PDE is a hyperbolic type and so has two sets of real characteristics. and that the characteristics are then for part (b) i used the coordinates: which leads to the equation: but then i am stuck... how can i get the 'general solution' from this, i think i am heading in the right direction, but i am not sure. any help would be greatly appreciated! thanks Sarah
8. ## wave equation on half-line! ahhhhhhh

but then how do you find a singularity when you have an answer of 1 and 0? :S
9. ## PDE problem : diffusion equation! help!

ok i got it now , thanks Bignose
10. ## wave equation on half-line! ahhhhhhh

lol my head is about to explode! i think this is similar to a previous question i asked but i can't quite get it none the less... now what i did was to following this : then using equations (31) & (32) from that i just plugged in the values and got: u(x,t) = 1 for x > ct and u(x,t) = 0 for 0 < x < ct how does this look to you intelligent mathematically inclined people? :S Sarah
11. ## wave equation pde problem

how do you know to add that? i see that it works, but at what point do you figure out to add it? that is it doesnt come out in any of the steps i take to solve the problem... does this mean for this problem: that i have to do a similar thing? (i.e. add $e^{x+ct}$ ? i get for that problem that u(x,t) = 1 for x > ct and u(x,t) = 0 for 0 < x < ct but that seems to simple, am i forgetting something completely obvious swansont? :S
12. ## wave equation pde problem

its ok i think its correct, its some other questions that i am having problems with now!
13. ## wave equation pde problem

ok, i think i did something like that...i said that part (a) has a solution of: $u(x,t) = f(x+ct)+g(x-ct)-\frac{x^3t}{6c^2}$ then using the initial conditions i solved for f and g... and came up with my answer of $u(x,t) = \frac{1}{6}xt^3$
14. ## PDE problem : diffusion equation! help!

ok. well i plugged my answer for part (a) back in and it works out, although i think what i have got as an answer could/should be simplified... leaving the answer as an integral looks a bit messy. however for part (b) i am totally stumped... how do you work with the BC that u(0,t) = 1 ?? any suggestions?
15. ## wave equation pde problem

i can't see how to apply seperation of variables (at least for part (a)).... i get this after plugging u(x,t) = X(x)T(t) into the orginal PDE: XT'' = (c^2)X''T + xt but you can't seperate the x's and t's ...? lol i am probably just completely wrong!
16. ## PDE problem : diffusion equation! help!

hmm ok , its just that my textbook says nothing about seperation of variables, all it talks about is how to get an explicit solution (such as my one above), which works in maple... i am little confused.... is my answer incorrect or...? i don't think seperation of variables works for part (a) because i end up getting that X(x) = 0 (because X(+inifinity) = exp(-x))

yes they are
18. ## PDE problem : diffusion equation! help!

hmm ok i tryed some more and i come up with this answer: however i don't know how to simplify it from here, i have looked up integral tables and still no luck. any suggestions guys? lol assuming the answer is right in the first place! cheers -sarah
19. ## PDE problem : diffusion equation! help!

Hi all, I am stuggling with this question ... so far i have only tried part (a), but since i can't see how to do that so far... ok so what to do... do we first look at an 'associated problem' ? ... something like lol, this stuff is all quite confusing -Sarah
20. ## wave equation pde problem

ok, i tried again and this is what i get... i included a non-homogenous term... for (a) for (b) what do you guys think?
21. ## wave equation pde problem

Hi everyone, I'm having a bit of trouble with this pde problem: i get the answer to be u(x,t)=0 but i am guessing thats not right. is the general solution to this problem: u(x,t) = f(x+ct) + g(x-ct) ?? thanks sarah

23. ## help! stokes theorem - integral problem

ah ok, that would explain why i have never heard my lecturer mention them

nm, got it!
25. ## Surface Integral of a sphere (or part there of :P!)

Hi all, would anyone be able to give me a little bit of help with this question. for part (a) i did this.... our sphere is given by $x^2 + y^2 +z^2 = 25$ putting in z = 3 to find out part of the sphere gives: $x^2 + y^2 = 16$ so we change to the parametric representation of the part of the sphere we are interested in sphere which is $\vec{r}(\theta,\psi) = (4sin\psi cos\theta) \vec{i} + (4sin\psi sin\theta) \vec{j} +(4cos\psi) \vec{k}$ then $\vec{r_\theta}(\theta,\psi) = (-4sin\psi sin\theta) \vec{i} + (4sin\psi cos\theta) \vec{j}$ $\vec{r_\psi}(\theta,\psi) = (4cos\psi cos\theta) \vec{i} + (4cos\psi sin\theta) \vec{j} +(-4sin\psi) \vec{k}$ so we get that $\vec{r_\theta}(\theta,\psi) \times \vec{r_\psi}(\theta,\psi) = -16sin^2\psi cos\theta)\vec{i} - 16sin^2\psi sin\theta \vec{j} -16sin\psi cos\psi \vec{k}$ our unit normal vector is given by: $\vec{\hat{n}} = -\frac{\vec{r_\theta}(\theta,\psi) \times \vec{r_\psi}(\theta,\psi)}{|\vec{r_\theta}(\theta,\psi) \times \vec{r_\psi}(\theta,\psi)|} = \frac{16sin^2\psi cos\theta)\vec{i} + 16sin^2\psi sin\theta \vec{j} + 16sin\psi cos\psi \vec{k}}{|\vec{r_\theta}(\theta,\psi) \times \vec{r_\psi}(\theta,\psi)|}$ does this look like i am heading in the right direction for part (a)? cheers! -Sarah
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