If you look around, you can find this worked out in a DE book or on line.
Here's another guy from Brazil that posted exactly the same question 2 minutes after you did. He received a (great!) reply.
See Conduction of Heat.
Maybe TPH did it also... "better than the book".
See http://www.mathhelpforum.com/math-he...-equation.html. This thread is worth to be a sticky one I believe.