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Math Help - Challenging Ladder Problem

  1. #1
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    Challenging Ladder Problem

    Hi,
    This is a very challenging problem. I've been working on this for a while. (it was a word document at first).

    Thanks. Any help is greatly appreciated
    Attached Thumbnails Attached Thumbnails Challenging Ladder Problem-capture3.jpg  
    Last edited by Jonanovi; February 17th 2008 at 08:43 PM.
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  2. #2
    Newbie DavePercy's Avatar
    Joined
    Feb 2008
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    The first thing to do is give names to some of the unlabeled distances in the picture. Lets call the height of the short ladder h_1 and the height of the long one h_2. We can also divide d into two parts where it intersects the dotted red line; call the one on the left d_1 and the one on the right d_2.

    Now we have two pairs of similar right triangles; each ladder forms a triangle with the building and with the dotted red line. We can set up some proportions for these triangles:

     \frac {d} {h_1} = \frac {d_2} {c}

     \frac {d} {h_2} = \frac {d_1} {c}

    By multiplying both sides of each equation by c and seeing that d_1 + d_2 = d we have
    \frac {d c} h_1  + \frac {d c} h_2 = d
    which after dividing both sides by d c becomes
    \frac 1 h_1  + \frac 1 h_2 = \frac 1 c

    Now all we have to do is use the Pythagorean Theorum to get h_1 = \sqrt {20^2 - d^2} and a similar expression for h_2 and substitute.

    The easiest way to do part b is to plug in 12 for d, then simplify the left side before taking the reciprocal of each side.

    Part c is basically guess and check; use a calculator or computer program to plug lots of two-decimal-place values into that expression until you can change the sign by changing the plugged in value by only .01.
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