Lagrangians of the (2+ 1)dimensional KP equation with variable coefficients and cross terms

Zhang constructed a Lagrangian for the (2 + 1)-dimensional KP equation with variable coefficients and cross terms [L. H. Zhang, Appl. Math. Comput., 219 (2013), 48654879]. This paper suggests a simple method to construct a needed Lagrangian using the semiinverse by introducing a simple auxiliary function, the presented method is simpler than Zhang s method to construct a Lagrangian.