Surface: z = 6-(x/8)^2-(y/8)^2
at points: A = (4,6,5.1875), B = (0,16,2)
The vector from pt. A to pt. B is <-4,10>.
The slope in the direction of x, (fx(x,y)), is -x/32.
The slope in the direction of y, (fy(x,y)), is -y/32.
The gradient of f(x,y) = z is -(1/32)(xi+yj).
The gradient of f(4,6) = z is -(1/16)(2i+3j).
The unit vector from pt. A to pt.B is
u = (1/116)^(1/2)(-4i+10j).
The directional derivative of z at (-1,-1) in
the direction of u is
(1/116)^(1/2)(-4i+10j) . -(1/16)(2i+3j)
= -0.1276695545.
