The scalar wave equation is  

(a) Find a general solution to this equation by separating variables in the form u(r)T (t).
(b) Use Cartesian variables to separate the equation satisfied by u(r). Write the general solution again.
(c) How does your final general solution for u(r, t) differ from the expression derived in the text for a wave packet composed of monochromatic plane waves? Show that this difference disappears for Re u(r, t).

2 . - zI?