2 (a) Use the method of separation of variables to derive a solution of the following initial-boundary-value problem:
utt(x, t) - c^2 uxx(x, t) = 0, 0 < x < L, 0 < t,
u(0, t) = u(L, t) = 0, 0 ≤ t,
u(x, 0) = ϕ(x), ut(x, 0) = ψ(x), 0 ≤ x ≤ L,
where L, c are given positive constants and ϕ, ψ are given functions.
(b) Find the solution u(x, t) of part (a) when
ϕ(x) = 5 sin(3π/L x), ψ(x) = 0, 0 ≤ x ≤ L.
(c) Find the solution u(x, t) of part (a) when
ϕ(x) = 0, ψ(x) = 2 sin(4π/L x), 0 ≤ x ≤ L.
(d) Use your solutions to parts (b) and (c) to write down the solution u(x, t) of part (a) when
ϕ(x) = 5 sin(3π/L x), ψ(x) = 2 sin(4π/L x), 0 ≤ x ≤ L.
(e) Let u(x, t) be the solution of part (b) and recall that the formula for u(x, t) models wave propagation on a string of length L. For what values of x0 ∈ [0, L] do we have that u(x0, t) = 0 for all values of t ≥ 0 ? What is the significance of these values of x0 from the point of view of the waves on the string?