3. Find the critical point of f(x,u) = xy + 2x - In x2y in the open first quadrant (x ＞ 0, y ＞ 0) and show that f takes on a minimum there.

5. A passenger ship and an oil tanker left port sometime in the morning; the former headed north, and the latter headed east. At noon, the passenger ship was 40 miles from port and sailing at 30 mph, while the oil tanker was 30 miles from the port and sailing at 20 mph. How fast was the distance between the two ships changing at that time?