申論題

6. (25%) Let u-u(x，y，t) be a function of the space (x.y) and the time t. S=S(s,f) is a function of x and t, and cl and co are constant. Determine whether the method of separation of variables is applicable to the following partial differential equations or not (each has 5 %). The rationale behind your answer needs be given; otherwise, it will not be credited.