Question

Prove that the sphere x^2+y^2+z^2=a^2 and the cone x^2+y^2-z^2=0 are orthogonal at every point of intersection.

Answer 1

I know we probably show that he dot product between those two surfaces is zero but the problem is that we can only use dot product on vectors.

I presume I find the gradient vector and then take the dot product?