⑩天掌 Teaching Plan on Advanced Mathematics o aP aP apaP 即 dzdx- dxdy= az ∑ ay az E00 ap aP PIx,v,f(x,y=+of OP aP dzdx -dxd PIx,y, f(x, y)lardy tianjin polytechnic dmivendityTianjin Polytechnic University Teaching Plan on Advanced Mathematics f dxdy z P y P dxdy y P dzdx z P y ( )   +   = −   −       即 P[x, y, f (x, y)]dxdy , y dxdy y P dzdx z P Dxy     = −   −    y f z P y P P x y f x y y    +   =   [ , , ( , )] 1