Can anyone help with a Maths quest?

Discussion in 'Homework Help' started by ElecNerd, May 15, 2011.

  1. ElecNerd

    Thread Starter New Member

    Jan 2, 2011
    Determine the minimum and maximum points on a graph?
    Y = 12cosθ - 5 sinθ with a range of 0 - 360°

    So far I have got...

    dy/dx = 12sinθ - 5cosθ

    sinθ/cosθ = -12/-5 = 2.4

    1st Quadrant = 2.4/Tan = Tan^-1 (2.4) = 67.38°

    The problem is I understand 360° is within the 4th Quadrant however by just adding 360 to 67.38° I get 427.38° - this can't be right...can it? How do I calculate the 4th quadrant so that I can then calculate the min and max values...

    Thanks alot for your help - from a very confused person :)
  2. steveb

    Senior Member

    Jul 3, 2008
    Well, you aren't all that confused because you have it mostly correct.

    There is a mistake in your dy/dx which should be -12cos-5sin, but you have the rest correct in principle. The only remaining thing is that arctan repeats every 180 deg, and not 360 deg. So just add 180 to your first answer to get the second answer.

    If your calculator gives you a negative value from the arctan then just add 180 degrees to get it in the range of 0 to 180. I think the standard range of arctan is -90 to 90 deg, but this is kind of an arbitrary mathematical convention which gets overridden by the question which demands the answer in the range of 0 to 360 deg. So your initial answer should be shifted by 180 deg until it is in the range of 0 to 180, and then add 180 deg to that answer to get the second answer.

    EDIT: I noticed another math error in your analysis. The final formula should be tan(x)=-5/12. So, you missed the sign and inverted the ratio. Instead of 12/5, it should be -5/12. But, you at least have most of the concepts down. We all make algebra mistakes like that.
    Last edited: May 15, 2011
  3. ElecNerd

    Thread Starter New Member

    Jan 2, 2011
    Thanks a lot for your help - its nice to get a good explanatory answer that does not make me feel silly - thanks again