(6-1) The separation of the space and time-dependent variables means that we can straightforwardly differentiate Ψ(x,t) with respect to time, as the ψ(x) term is not a function of time, and therefore behaves just like a constant in this operation:
① Differentiate twice to find ∂2Ψ(x,t)/ ∂t2
② Notice that you can express your answer in terms of Ψ. Do this.
③ Now apply the wave equation (equation VI-3) to find an expression for ∂2 Ψ (x,t)/ ∂x2
④ Deduce the expression for d2ψ(x)/dx2.