Don't you just love related rates?

[ms. confused]

The angle of elevation of the Sun is decreasing at a rate of 0.25 rad/h. How fast is the shadow cast by a 400-ft-tall building increasing when the angle of elevation of the Sun is π/6?

I thought about using the special triangle to find exact values for trig functions at π/6 but I don't know which equation I should use to relate the rate of change. What should I do?

 

[Tide]

Just write $L = H \cot \theta$ with the obvious definitions and differentiate.