I did the first part by the definition of total derivative, but can not think of the way to do the second part of the question. Any help please?
I meant differentiate with respect to x, using the chain rule. I suppose I should have used t rather than x, since the question says that f is a function of t. Then evaluated at t=a is equal to , and similarly for the other coordinates. The result of differentiating both sides of the equation in that way ought to remind you of an inner product.