Sphere x^2 +y^2 +z^2 =a^2 can be written as

r(phi, theta) = <asinphicostheta,asinphisintheta, acosphi>

0< or = phi < or = pi, 0< or = theta < or = 2pi.

Show that the radius vector r is normal to the tangent plane at each point on the sphere.

Thank you very much.