複選題9. (5%) Consider the wave equation. Which are the possible solutions? (A) u(x,t)=f(x-at) (B) u(x,r)=f(x+at) (C) (D) (E) None of the above

4. (5%) Consider the equationwiththe boundary conditions y(0)=0.y(π)=π.Which statements are correct? (A) The particular solution is yP =-x. (B) The general solution is y=ex sinx+x. (C) There is a unique solution to this boundary-value problem. (D) This differential equation is homogencous. (E) None of the above.