0zz xz y0),( yxfz PQMN x yAB),(000zyxM),(000zzyyxxN dz=AB : 切面立标的增量切面立标的增量z= f (x ,y)(d yxzz z =AN ：曲面立标的增量曲面立标的增量过点过点M的切平面的切平面：)(,()(,(000000yyyxfxxyxfyx 即即：dz z=AB+BN0 zz 0)(0 zzyyxfxyxfyx ),(),(0000)( .dz=AB用切面立标的增量近似曲面立标的增量用切面立标的增量近似曲面立标的增量 很很小小时时当当y,x zdz全微分的几何意义全微分的几何意义0zz xz y0),( yxfz PQMN x yAB),(000zyxM),(000zzyyxxN dz=AB : 切面立标的增量切面立标的增量z= f (x ,y)(d yxzz z =AN ：曲面立标的增量曲面立标的增量过点过点M的切平面的切平面：)(,()(,(000000yyyxfxxyxfyx 即即：dz z=AB+BN0 zz 0)(0 zzyyxfxyxfyx ),(),(0000)( .dz=AB用切面立标的增量近似曲面立标的增量用切面立标的增量近似曲面立标的增量 很很小小时时当当y,x zdz全微分的几何意义全微分的几何意义
