PROBLEM 4.3. The one-dimensional wave equation is
∂²u/∂t² - c²∂²u/∂x² = 0,
where c > 0 is constant. Show that any function of the form u(x,t) = f(x - ct) + g(x + ct), where f,g: ℝ → ℝ are twice continuously differentiable, satisfies this equation. Explain why we call c the wave speed.