u ' & ltx & gt=(-x^2+y^2+z^2)/(x^2+y^2+z^2)^2，

u ' & lty & gt= -2xy/(x^2+y^2+z^2)^2，

u ' & ltz & gt= -2xz/(x^2+y^2+z^2)^2，

In a (1, 2,2), u'

At point b (-3, 1, 0), u'

The gradient vector of point A (7, -4, -4) and the gradient vector of point B (-4, 3, 0),

Cosine cost of the angle between two vectors = [7 (-4)+(-4) 3+(-4) 0]/[√ (7 2+4 2+4 2) √ (4 2+3 2)] =-8/9.

t = arccos(-8/9) = π - arccos(8/9)