Both of the highlighted equations deal with a standing wave. However, they are slightly different in the sense that the latter has a phase shift in it. Why ? Also, how does one go from the latter equation for a standing wave to : 2A*sinkx*sinwt. For a string with both ends fixed And 2A*sinkx*coswt. For one end fixed and the other free And 2A*coskx*coswt. For both free.