Find the directional derivative of the function at P in
the direction of v.

f(x, y, z) = xy +
yz + xz,    P(1, 2,
−1),    v =
2i + j − k

Use Theorem 13.9 to find the directional derivative of the
function at P in the direction of

PQ.

f(x, y) =
e^10y
sin(x),    P(0,
0),    Q(4, 1)