Length of one side for maximum area of trapezoid (solution by Calculus)

Problem
BC of trapezoid ABCD is tangent at any point on circular arc DE whose center is O. Find the length of BC so that the area of ABCD is maximum.
 

Trapezoid with one side tangent to the circle

 

Solution
As described by Alexander Bogomolny of cut-the-knot.org, for maximum area of trapezoid, the point of tangency should be at the midline of AB and DC, thus H is the midpoint of BC.
 

032-largest-trapezoid-by-geometry.gif

 

17-18 A man in a motorboat needs to catch a bus

17-given-figure.jpgProblem 17
A man in a motorboat at A receives a message at noon, calling him to B. A bus making 40 miles per hour leaves C, bound for B, at 1:00 PM. If AC = 30 miles, what must be the speed of the boat, to enable the man to catch the bus?
 

Problem 18
Solve Problem 17, if AC = 20 miles and the bus makes 50 miles per hour, leaving C at 12:18 PM, bound for B.