... v`ao“ˆo trˆo´ng”.Tu.o.ng tu..d 3 f =∂∂xdx +∂∂ydy 3 f(x, y)=∂ 3 f∂x 3 dx 3 +3 ∂ 3 f∂x2∂ydx2dy +3 ∂ 3 f∂x∂y2dxdy2+∂ 3 f∂y 3 dy 3 ,v.v Mˆo.t c´ach quy na.p ta ... 1)=0,fx2(1, 1) = 0,fxy(1, 1)=1,fy2(1, 1) = 0,f (3) x 3 (1, 1)=0,f (3) x2y(1, 1)=1,f (3) xy2(1, 1) = 0,f (3) y 3 (1, 1)=0. 3 +Thˆe´v`ao cˆong th´u.c (*) ta c´odf (1, 1) = fx(1, ... 1)∆y2=2∆x∆y,d 3 f(1, 1) = 3 x2∆yv`a do d´oxy=1+∆x +∆x∆y +12∆x2∆y + R 3 . V´ı du.7. T´ınh vi phˆan cu’a h`am ˆa’n w(x, y)du.o..cchobo.’iphu.o.ngtr`ınhw 3 +3x2y +...