I need help with this problem in my calculus book. An airplane at an altitude of 10,000 feet is flying at a constant speed on a line which will take it directly over an observer on the ground. If, at a given instant, the observer notes that the angle of elevation of the airplane is 60 degrees and is increasing at a rate of one degree a second, find the speed of the airplane. (Hint: tangent of theta is equal to sine of theta over cosine of theta) I've worked it and had tangent of theta equals "x" over 10,000. And took the derivative of each side to get secant squared of theta equals one over 10,000 times dx/dt. I solve for dx/dt with my information and get 40,000 feet per second. However the boom says 10,000*pi over 135. I don't get where the pi comes from. Sorry for no LaTeX, I'm on my phone.