http://i36.tinypic.com/2s0bje2t.jpg

I was thinking about using the chain rule because

(Dh)_t = ((Dh)_a(t)) o (Da)_t where I believe ((Dh)_a(t))=(grad h)(a(t))

but couldn't really relate this to a dot product that would be 0.

Any help would be greatly appreciated. Thanks