my fuction is z=6-(1/2)(x^2)-(1/8)(y^2)
and my points are P(3,-3,3/8) R(0,0,6)
then I can find the vector V=(-3,3)
and unit vector U= V/IIVII
U=(1/(3(2)^(1/2))(-3i+3j)
=(1/(2)^(1/2))(-i+j)
because grad f(x,y)=fx(x,y)i+fy(x,y)j is
grad f(x,y)= -xi-(1/4)y
grad f(3,-3) = -3i-(3/4)
consequently, at (3,-3,3/8) the directional
derivatiive is
Duf(x,y)= (-3)(-1/(2)^(1/2))+(3/4)(1/(2)^(1/2))
= (12+3)/(4(2)^(1/2))
=(15(2)^(1/2))/8
