(4-4)    Case 1:   you have two identical waves, travelling in the same direction and in-phase.  This means that the peaks in one are exactly aligned with the peaks in the other. The equations describing the two waves are therefore the same:

           y1  =  y2  =  Asin(2π(x-vt)/λ)     

Deduce the expression for the superposed wave by calculating (y1 + y2) as a function of x.  

What will the resultant wave look like?