let f be a function of two variables that has continuous partial derivatives and consider the points a(8, 3), b(12, 3), c(8, 10), and d(14, 11). the directional derivative of f at a in the direction of the vector ab is 4 and the directional derivative at a in the direction of ac is 15. find the directional derivative of f at a in the direction of the vector ad. (round your answer to two decimal places.) 18.16 incorrect: your answer is incorrect.