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Consider the illustration, which shows a rotating beam of light located 0.5 mile from a shoreline. The beam rotates at a rate of 4 revolutions per minute. How fast (in miles per minute) is the distance between the beam and the point where it strikes the shore changing at the instant when x = .25 miles?

  • calculus -

    Believe it or not, my crystal ball is working to today and I can "see" your illustration.

    let the angle formed be Ø, then
    dØ/dt = 4(2π) or 8π rad/min

    tan Ø = x/(1/2)
    tanØ = 2x
    sec^2 Ø dØ/dt = 2 dx/dt
    dx/dt = (sec^2 Ø)(dØ/dt)/2

    when x = 2.5, tan Ø = 2.5/.5 = 5/1
    then cosØ = 1/√26
    secØ = (3x^4 + 6x^2 - 48x + 8)26
    sec^2 Ø = 26

    dx/dt = (26)(8π)/2 = 1004π miles/min

  • calculus -

    third last line should have been

    sec Ø = √26

    (on my Mac, I create √ by pressing "option" V, but I must have pressed "command" V, giving me a "cut-and -paste" of some previous post)

  • calculus -


    dØ/dt=4 * 2π = 8π rad/min

    x=0.5 tan(Ø)
    dx/dt=0.5 sec^2(Ø) dØ/dt
    dx/dt=(x^2+0.5^2)/0.5 * 8π

    when x=0.25 miles

    dx/dt=(0.25^2+0.5^2)/0.5 * 8π = 5π = 15.7 miles/min

    (note to about attempt: if it were x=2.5, then dx/dt=104π miles/min)

  • calculus -

    Oops! Third line should be:


  • calculus -

    Um ...


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