f(x, y, z) = x^{2} + y^{2} + z^{2} − R^{2} = 0
normal = \frac{∂f}{∂x} x̂ + \frac{∂f}{∂y} ŷ + \frac{∂f}{∂z} ẑ = 2xx̂ + 2yŷ + 2zẑ