Related Rates Lighthouse

Related rate problem involving rotational motion. Example from Calculus Stewart 6th Edition

[alialshubbar]

you saved my GPA, typically in a 12 questions exam, 4 of which are related rates problems

[roxannecoffey]

I kept trying to get theta and got so stuck. Thank you!

[mibo]

how did the units change from radians to km on top? isn't it radians/min instead of km/min?

[skeptical]

I got dx/dt = 4pi/5 since 1 revolution of a light beam along the circumference of a circle of radius 3 is = to 6pi. I understand how you arrived at your answer using a standard unit circle of radius 1 with circumference 2pi .

But, why am I wrong?

[avacairns]

thank you so much. How do you film and upload these videos?

[mohammedalnayar]

a revolution means that it did 360 degrees.. and thats the circumference of a circle. so why didnt you use (2 pi r) instead?