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Homework Help: Need to determine distance maximizing angle theta.

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data

    A movie screen at the front of a room is 9 feet above the ground and 16 feet tall. Determine the distance X you must sit away from the screen in order to maximize your viewing angle.

    [ I tried to make the picture :P. I need to find X]

    http://img50.imageshack.us/img50/2781/probwk6.png [Broken]

    The little fish thing is i believe alpha...i don't know the Greek name for that.

    2. Relevant equations

    Well i figured that i need to do the tangent of both triangles and set them equal to each other, but i am confused if i need to take into account the second triangle for tan of theta.

    I am comming up empty handed, and what i got was 25/tan[theta] but i don't think its right.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 30, 2008 #2
    help...anyone? i kinda need desperate help :P
  4. Oct 31, 2008 #3


    Staff: Mentor

    Good drawing! And, yes, that Greek letter is alpha.

    What you need is an equation with theta as a function of x.

    First, let's take care of alpha: cot(alpha) = x/9, so alpha = cot^(-1)(x/9).

    Next, let's get a relationship between the larger angle and x.

    tan(theta + alpha) = 25/x
    so theta + alpha = tan^(-1)(25/x)

    But we already have alpha, so you should be able to get theta as a function of x; i.e., theta = f(x).

    Now, do the usual and find d(theta)/dx and set it equal to 0.

    Does that get you started?
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