Spo�t�te $\displaystyle\triangle u=\frac{\partial^2
u}{\partial x^2}+ \frac{\partial^2
u}{\partial y^2}$, jestli�e $u =
f(x+y+z,x^2+y^2+z^2),$ kde $f$ je nekone�n� diferencovateln�
funkce na $\RR^2$.
