Use the method of separation of variables to derive the solution of the following initial-boundary-value problem:
utt(x, t) - c^2uxx(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 c is a given positive constant and ϕ, ψ are given functions.
What assumptions would you make about the functions ϕ, ψ in part (a) in order for your solution to part (a) to be valid?
Write down the solution u(x, t) to part (a) when the functions ϕ, ψ are given by the following formulas:
ϕ(x) = 0, ψ(x) = 4 sin(2πx/L).
Use standard trigonometric identities to write the solution u(x, t) that you found in part (c) in the form
u(x, t) = f(x - ct) + g(x + ct)
where f and g are a suitable pair of functions.