2. Figure 3.7 plots a harmonic plane wave at t = 0, traveling in the x direction at 5 km/s. (a) Write down an equation for this wave using the sine function that describes displacement, u, as a function of x and t. (b) Write the corresponding equation using the cosine function. (c) What is the displacement at x = 6 km and t = 30 s? (d) What is the maximum strain for this wave? 8 km V=5 km/s 0.04 m