17. Let f(x, y) 1+x+y+x+axy + y be a function of two variables. Suppose it is always true that af x = What is the value of a? af (a) 1 (b) 2 (c) There is no such value of a since the partial derivatives must be different. (d) 0 (e) Any value of a is possible since mixed partial derivatives are equal. 18. A stock analyst tracks three companies with stock prices f(t) = 40+t+t2- t/10, g(t)=7+t-t2 + 3/5, and h(t) 15+tt2/2+t/10 - 14/200, t months into the year. A monthly buy recommendation is due at the end of August, so t = 8. The analyst believes in 'momentum investing' and looks for companies with stock prices that are concave upwards at the time of the recommendation. Which company is recommended? (a) The company with stock price h(t). (b) The company with stock price g(t). (c) Neither of the three tracked companies. (d) All three of the tracked companies. (e) The company with stock price f(t).