# Homework Help: Hi every one(i'm remine)

1. Oct 29, 2006

### ريمان

hi

are you fine all i hope that

today my son give me this problem to solve it but i cant

can you solve Differential Equations

are you can help me to solve it please

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2. Oct 29, 2006

### Integral

Staff Emeritus
I have to admit that I am a bit curious. Why would someone taking a course in PDEs bring home problems for Dad, and why would you have to come here to post a question for your adult child? (I assume adult, since generally PDEs are not presented before the 2nd or 3rd year in a university.

What you have there is the Heat equation. The solution to this is a bit involved, perhaps your child needs to post what s/he knows. Surely s/he is taking a class and should have learned something there.

3. Oct 30, 2006

### ريمان

hi master integral

my son is in secondrey school and he try to be good in math and he love math that he try to understan the Differential Equations

can you help me to solve it

4. Oct 30, 2006

### HallsofIvy

Then I guess the question is "How good are you at differential equations". I can imagine having a son in secondary school that is working on partial differential equations can be rough! There is a standard method for solving the heat equation: separation of variables. If you let U(x,t)= X(x)T(t); that is, assume U can be written as the product of two functions, one, X, depending only on X and the other, T, depending only on t, then Uxx= X"T and Ut= XT' (' and " are first and second derivatives with respect to the appropriate variable). The equation becomes XT'= KX"T. Dividing through by XT gives
T'/T= K X"/X. That is, the left side depends only on t and the right only on x. If we vary x while t remains the same, the left side doesn't change- therefore the right side can't either: K X"/X must be a constant. Similarly, the left side must be that same constant for all t. Call that constant C. We now have the two ordinary differential equations T'/T= C or T'= CT and K X"/X= C or KX"= CX. If your son cannot solve those equations he should go back and study ordinary differential equations before he gets into partial differential equations! The first has the obvious solution T= AeCt. The solution to the second depends upon whether C is positive or negative. If C is positive then the solution will involve exponentials and will be unbounded at either + or - infinity so C must be negative. In that case, the solution will be in terms of sine and cosine or, equivalently, complex exponentials. In general you will need a sum (more correctly, an integral) of such things: the Fourier Transform.

Before doing a problem like this, your son should be comfortable with ordinary differential equations, partial differential equations over finite integrals, whose solutions involve Fourier series, and the beginnings of Fourier Transform theory. On the other hand he could just go outside and play soccer and wait for college.

5. Oct 30, 2006

### ريمان

hi tank you my lord

yes my son try to be btter and he try to slove hard things

6. Oct 30, 2006

### Maxwell

Nice try. :rofl: