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Homework Help: Trig Proof

  1. Mar 18, 2009 #1
    1. The problem statement, all variables and given/known data
    So you basically have to prove this big long equation d=2v2cos(o)sin(O+o)/gcos(O)

    2. Relevant equations

    3. The attempt at a solution

    so i have weeded it down to this:


    so i have the sin(O+o)=sin(O)cos(o)+sin(o)cos(O) part but i dont know how to get rid of the denominators and to get the other parts of the equation!
  2. jcsd
  3. Mar 19, 2009 #2


    Staff: Mentor

    Are you trying to prove that the equation is an identity, or are you trying to solve the equation? I don't think you're actually trying to prove this is an identity, but I have no idea what you're trying to solve for.

    BTW, you're setting yourself up for disaster by choosing o and O for variables, both of which look a lot alike, and both of which resemble 0 (zero).

    I would strongly advise changing your variables to something like this:
  4. Mar 19, 2009 #3
    you're trying to prove the distance
    and yeah well those were just the variables that were given to us but you're right it would be a lot easier to use a and b or something

    any idea on how to solve?

    basically the question he gave us is:
    A cannon ball is fired from an angle o with an initial velocity of v. The hill slopes own with an angle of O. Prove that the horizontal distance the cannon ball travels is given by d=2v2cos(o)sin(O+o)/gcos(O)
  5. Mar 19, 2009 #4


    Staff: Mentor

    No, you're not trying to prove the distance, which makes no sense. You're trying to prove that the distance can be obtained from the formula you showed. IOW, if the gun is fired as described, you have to come up with the formula d=2v2cos(a)sin(b+a)/gcos(b). You don't start from it; you end up with it.

    How is this distance measured - horizontally or along the ground down the hill? It makes a difference.

    You're going about this the wrong way. Based on the given information, you should end up with the formula for distance. What you seem to be doing is trying to use trig identities to write the formula in a different way. That is not what the problem is asking you to do.

    Have you drawn a diagram? I can guarantee that you will have no success without a drawing.

    What are the forces on the round after it leaves the barrel of the gun?
    Again, change the variables, with o = a, and O = b.
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