(4-7) Recall, from the previous section, that our expression for a wave moving to the right is:   y = f(x – vt)

①  First make a substitution:  let q = x – vt.  Use the chain rule to find expressions for  ∂y/∂x and ∂y/∂t, in terms of  dy/dq.

②  Now write down the relationship between the two partial derivatives.