11.
	a)
		f(r.)=(4x^2+4y^2+4z^2+5)^(-1)
		
		r(f)=sqrt(1/4f -5/4) *n
		r(0,2)=0
		r(0.15)=0,645 (sqrt(15)/6)
		r(0,05)=1,936 (sqrt(15)/2)
		(1/3)
		
		a.(r.) = f.(r) = (4x,4y,4z)*-(4x^2+4y^2+4z^2+5)^(-2) = -4r./(4r^2+5)^2 

	b)	
		nabla * a.(r.)= div(a.(r.))=(-4(4r^2+5)^2+4r.*8r*2(4r^2+5))/(4r^2+5)^4
		nabla x a.(r.)= rot(a.(r.))=()