If "f" is a real-valued function on Rn, then the partial derivatives of "f" measure its variation in the direction of the coordinate axes. For example, if "f" is a function of "x" and "y", then its partial derivatives measure the variation in "f" in the "x" direction and the "y" direction. They do not, however, directly measure the variation of "f" in any other direction, such as along the diagonal line . These are measured using directional derivatives. Choose a vector