ex1.求z=x2+3xgy+y2在点(1,2)处的偏导数 Methodl. c =2x+3y; 03x+2y ax 0a ≈2×1+3×2=8, z x=1=3×1+2×2=7 Method2.∵:z(x,2)=x2+6x+4,z(1,y)=1+3y+y2, 2/=1=(2x+6) 8 2 y=1=(3+2 y 少2≈7 K心ex1. 求 2 2 z = x + 3xy + y 在点(1,2) 处的偏导数． Method1. =   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 . Method2. ( ,2) 6 4, 2 z x = x + x + (1, ) 1 3 , 2 z y = + y + y =    = = 2 1 y x x z (2 6) 8, 1 + = x= x =   = = 2 1 y x y z (3 2 ) 7. 2 + = y= y