Exercise 35

Let f be a function of two variables that has continuous partial derivatives and consider the points A(1, 3), B(3, 3), C(1, 7), and D(6, 15). The directional derivative of f at A in the direction of the vector \(\vec{AB}\) is 3 and the directional derivative at A in the direction of \(\vec{AC}\) is 26. Find the directional derivative of f at A in the direction of the vector \(\vec{AD}\).