I have to take the FE in April and haven't had a math class since 2001, so can someone please jog my memory on this one:
How fast is the distance of an airplane from a fort increasing 1 minute after passing directly over the forst at a height of 10,000 feet when it is flying horizontally at the rate of 200ft/sec?
I used the right triangle with side a = 10,000 ft and side b = 12,000 ft (from 60 sec * 200 ft/sec). C is the distance of the plane from the fort.
c = sqrt(a^2 * b^2). I solved for dc/db, is this correct? I ended up with:
dc/db = b
---------------- .
sqrt(a^2 + b^2)
If I plug and chug from here, I don't get the answer given in the book. However, if I use b^2 in the numerator, then I get the right answer, but how does the numerator become b^2?