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Math Help - Sliding ladder problem

  1. #1
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    Sliding ladder problem

    So this is an assigned problem, but I can assure you that I have tried this problem multiple times and still seem to get the incorrect answer.



    If we suppose the unknown rate of change in the angle as z...

    x/8 = tan z
    x = 8 tan z
    dx/dt = 8 sec(z)^2 * dz/dt
    dz/dt = 1/8 cos(z)^2 * dx/dt
    dz/dt = 1/8 cos(z)^2 * 1.3
    dz/dt = 1.3/8 cos(z)^2
    dz/dt = 1.3/8 (8/10)^2 <-- cos(z) is 8/10
    dz/dt = 1.3/8 (64/100)
    dz/dt = 0.104 rad/s

    But it still says I'm wrong...? I've done the problem from the beginning a few times, and I still get the same answer. HELP!
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by RaDeuX View Post
    So this is an assigned problem, but I can assure you that I have tried this problem multiple times and still seem to get the incorrect answer.



    If we suppose the unknown rate of change in the angle as z...

    x/8 = tan z
    x = 8 tan z
    dx/dt = 8 sec(z)^2 * dz/dt
    dz/dt = 1/8 cos(z)^2 * dx/dt
    dz/dt = 1/8 cos(z)^2 * 1.3
    dz/dt = 1.3/8 cos(z)^2
    dz/dt = 1.3/8 (8/10)^2 <-- cos(z) is 8/10
    dz/dt = 1.3/8 (64/100)
    dz/dt = 0.104 rad/s

    But it still says I'm wrong...? I've done the problem from the beginning a few times, and I still get the same answer. HELP!
    actually, tan z = y/x. but you wouldn't want to use that equation.

    cos z = x/10 is better. try with that

    (by the way, plugging in 8 right away is bad. not the right thing to do. if something is changing, you don't plug in a constant to begin with, because it will have a rate that shows up upon implicit differentiation. use variables to represent things that change. only plug in values if they are constant and/or you want to solve for something after doing all your differentiation and mumbo jumbo. also, per the question, it would be x = 8 as opposed to y = 8 for the instant in question).
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  3. #3
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    I re-did everything. Apparently I mixed up x with an unknown number for y (not sure why I did that).

    I entered -0.216, and it still said it was wrong. I get up to five guesses on the problem, so I'm not sure if I was supposed to round up to -0.217.
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