要使此方程不含一阶偏导数项则只需 2a+k=0 -2B+k=0 B →a2-B2+ko+kB=0 0.15 要使此方程不含一阶偏导数项.则只需 2 0 2 0 k k    + =   − + = , . 2 2 k k  = − =   2 2  − + + =     k k 0 2 2 2 2 0. v v x y    − =  