instantaneous power

Instantaneous Power in a Standing Wave. From Eq. (15.21), the instantaneous rate at which a wave transmits energy along a string (instantaneous power) is


Where F is the tension. 
(a) Evaluate f (x, t} for a standing wave of the form given by Eq. (15.28). 
(b) Show that for all values of x, the average power Pav carried by the standing wave is zero. (Equation (15.25) does nut apply here. Can you see why?)