A balloon rises at the rate of 8 feet per second from a point on the ground 60 feet from an observer. Find the rate of change of the angle of elevation when the balloon is 25 feet above the ground.

My work:

picture: http://i41.tinypic.com/14j0vuv.jpg

tan Θ=y/x

tan Θ=25/60

Θ = 22.78 degrees

derivative of tanΘ=y/x:

sec˛Θ(dΘ/dt) = [ (x)(dy/dt) - (y)(dx/dt) / x˛ ]

1.176(dΘ/dt) = [ 60(8) - (25)(19.2) / 60˛ ]

^^I got the dx/dt number, 19.2 by setting up this proportion:

60/x = 25/8

But then I realized that when I solve this: 60(8) - (25)(19.2), I get 0...what am I doing wrong??

**EDIT: Sorry, you're right, I meant dx/dt and dy/dt

Thanks!