7.3 Exercise 11

A particle with mass m is described by the following wave function: $\psi \left(x\right) = A \text{cos} k x + B \text{sin} k x$, where A, B, and k are constants. Assuming that the particle is free, show that this function is the solution of the stationary Schrӧdinger equation for this particle and find the energy that the particle has in this state.