面S : z+x^2+y^2=a^2
　dz+2xdx+2ydy=0
(2x,2y,1)(dx,dy,dz)
|(2x,2y,1)|^2=4x^2+4y^2+1=4(a^2-z)+1

n=(2x,2y,1)/(4a^2+1-4z)^(1/2)