例1求z=x2+3x+y2在点(1,2)处的偏导数 解 z =2x+3y; ax =3x+2] or=1=2×1+3×2=8 9 y=2 az x=1=3×1+2×2=7 y=2 上页例 1 求 2 2 z = x + 3xy + y 在点(1,2) 处的偏导数． 解 =   x z 2x + 3y ; =   y z 3x + 2y . =    = = 2 1 y x x z 21+ 3 2 = 8 , =   = = 2 1 y x y z 31+ 22 = 7