14. Using the Lagrangian density 1 where V'(u)= sin u, derive the sine-Gordon equation %3D "-u = sin u. 15. Show that the bilinear form of the sine-Gordon equation in Exercise 14 is the pair of equations %3D (D - D -1)(*-w)=0 and (D - D})(+0- w w) = 0, (). where u = 4 tan