# Heat equation problem so confusing

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1. Dec 17, 2014

### JI567

1. The problem statement, all variables and given/known data
The problem is f(x) = sin2πx - (1/πsquare)*sinπx

and its given Bn sin (nπx) = f(x)

Question is find Bn.

2. Relevant equations

Bn = 2/L ∫ (sin2πx - (1/πsquare)*sinπx) * sin(nπx/L) where L is 1
3. The attempt at a solution
I did

∫ sin2πx * sin (nπx) - (1/πsquare)*sin square πx
then I tried changing it to cos but it doesn't make sense, for the first term ∫sin2πx * sin (nπx)
∫sin2πx * sin (nπx) = -1/2 ∫cos(2π+nπ)-cos(2π-nπ)

But how do i actually integrate the above with the n in it i am so confused.....I know cosnπ = (-1)^n but I can only use that after final integration and even if I finally somehow manage to integrate it, it will be changed to sinnπ. Please can anyone help!!!
. Is there any simple way to do this?

2. Dec 17, 2014

### haruspex

That doesn't make sense. Please correct the problem statement.

3. Dec 17, 2014

### Staff: Mentor

Did you mean to have a summation sign in front of the Bnsin(nπx)?

4. Dec 17, 2014

### Ray Vickson

Your question seems to be asking for the $B_n$ in
$$\sum_{n=1}^{\infty} B_n \sin(n \pi x) =\sin(2 \pi x) - \frac{1}{\pi^2} \sin(\pi x)$$

5. Dec 18, 2014

### JI567

Yesss! I didn't know how to insert the symbols like that, but yes that's exactly my question. Is there any simple way to find Bn? Many thanks

6. Dec 18, 2014

### Staff: Mentor

Just write out the first two terms of the summation on the left hand side and see what you get.

Chet

7. Dec 18, 2014

### JI567

Sorry how do i do that? is it just Bn Sin(πx) + Bn sin (2πx)

I just used n =1 and n =2. Is that correct?

8. Dec 18, 2014

### Staff: Mentor

No. It's B1sin(πx)+B2sin(2πx).

Now compare that with the right hand side of your equation.

Chet

9. Dec 18, 2014

### JI567

So B1 = -1/π square and B2 = 0, right? So do I just take Bn as -1/π square then? Isn't Bn a general constant for all the B values in the summation? I am just confused how two different B values can have the similar Bn value.

10. Dec 18, 2014

### JI567

Sorry I meant B2 = 1

11. Dec 18, 2014

### Staff: Mentor

No. You take B1=-1/π2, B2 = 1, B3...B = 0
I don't understand this question.

Chet

12. Dec 18, 2014

### JI567

You know heat equation solution is in a format of T(x,t) = Bn*sin(nπx)*e^-(nsquare*pi square* t) - T(x). So you are saying I will have one T(x,t) equation with B1 and another T(x,t) equation with B2? Do I add up both the equations together to get a final T(x,t) solution.

13. Dec 18, 2014

### Staff: Mentor

You seem very confused. This is not the proper venue to teach you the entire solution approach over again. You need to go back and review your notes and text.

Chet

14. Dec 18, 2014

### JI567

O
No I am just confused which format should I present my answer. Do I just put the summation sign infront of Bn and then write when n = 1 bn = -1/pi square. And when n = 2 Bn= 1.

15. Dec 18, 2014

### Ray Vickson

When you write "nsquare" do you mean n^2? By pi square do you mean pi^2? Where is the summation in front of your nth term? (In text you can just write sum_{n} B_b... if you don't want to try using LaTeX and you don't want to use fancy symbols.

16. Dec 18, 2014

### JI567

Yes exactly that's what I mean. And yes there will be the summation sign, apologies. So is that it then? I just write T(x,t) = sum_{n}B_n *sin(nπx)*e^-(n^2*pi^2* t) - T(x). Then I just mention when n =1 and 2 there will be corresponding B values. Will that be the final answer?

17. Dec 18, 2014

### Ray Vickson

Why bother with the summation? You just have two terms, so just writing them out in detail answers the question.

18. Dec 18, 2014

### LCKurtz

To the OP: Setting your heat equation problem aside, what you need to understand is that some FS are finite. For example if you want to expand $f(x) = 5\sin(3x) + 3\sin(4x)$ in a half range sine expansion on $(0,\pi)$ you are wanting to write$$5\sin(3x) + 3\sin(4x) = \sum_{n=1}^\infty b_n \sin(nx) = b_1\sin(x)+b_2\sin(2x) + b_3\sin(3x)+b_4\sin(4x)+...$$As you noticed in your original post, solving for the $b_n$ can be very tedious and requires working the integrals for $b_3$ and $b_4$ separately. Actually, I don't think you got that far in those calculations. But looking at the above equation you can see by inspection that taking $b_3=5,~b_4=3$ and all the other $b_n=0$ makes the two sides identical. The FS for this function has finitely many nonzero terms, and in fact, the function itself is its own FS.

Similarly, in your solution, you will have only a couple of nonzero terms in your FS and consequently won't need any infinite sum in your answer. Just write out the nonzero terms.

19. Dec 18, 2014

### JI567

Alright so my solution will be like this :

T(x,t) = b1sin(πx)*e^-(n^2*pi^2* t)+b2sin(2πx)*e^-(n^2*pi^2* t) - T(x).

Is that correct for the final answer?

20. Dec 18, 2014

### LCKurtz

No. You have undefined $n$'s on the right side and undefined $b_1$ and $b_2$ on the left.