# Reflection Principle(PDES)

1. May 8, 2009

### Winzer

Can someone explain to me why do we use this for boundary conditions in the heat, and wave equations? Why must we make an odd extension of our initial data and not even?

2. May 9, 2009

### matematikawan

Probably because integration of an odd function is zero.

$$\int_{-a}^a f(x) dx = 0$$