练习15 1.取I={-1,0,0}, af af cos a+cos B+cosy al ax oz ax 七∫x(xo,n)c+fxn,yn)sia1=1 ∫x(x0,J0)c0s的2+∫(x0,Jo)sin62=0 ∫x(x0,y0)+ 2 2y(x0,)=1 ∫x(xo,y0)=3 ,(xn) 2 ∫x(x0,y)+。J (x0,y0)=0 2 grad(x0,y)=√3-j,∴ gradf(xo,y)=2即为所求 K心练习1.5 一. 1. l = {−1,0,0},  取 cos cos  cos z f y f x f l f   +   +   =   而 . x f   = − 七. , ( , )cos ( , )sin 0 ( , )cos ( , )sin 1 0 0 2 0 0 2 0 0 1 0 0 1    + = + =     f x y f x y f x y f x y x y x y , ( , ) 0 2 3 ( , ) 2 1 ( , ) 1 2 1 ( , ) 2 3 0 0 0 0 0 0 0 0        + = + = f x y f x y f x y f x y x y x y , ( , ) 1 ( , ) 3 0 0 0 0    = − = f x y f x y y x ( , ) 3 , 0 0 gradf x y i j    = − ( , ) 2 .  gradf x0 y0 = 即为所求