Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Solving a Trig Problem - is this iterative only?

  1. Mar 18, 2008 #1
    1. The problem statement, all variables and given/known data

    Find when this is "0" for the first time:

    [tex] q(t) = e^{-20t} (5cos(40t) + \frac{5} {2} sin(40t)) [/tex]


    2. Relevant equations



    3. The attempt at a solution

    [tex] 0 = e^{-20t} (5cos(40t) + \frac{5} {2}sin(40t)) [/tex]

    [tex] 0 = (5cos(40t) + \frac{5} {2}sin(40t)) [/tex]

    [tex] cos(40t) = -\frac{1} {2}sin(40t)) [/tex]


    Is the best way to solve this perform interations?

    Thanks
    Sparky
     
    Last edited: Mar 18, 2008
  2. jcsd
  3. Mar 18, 2008 #2
    Iterations on what? You've basically solved it. Make that into [itex]tan(40t) = -2[/itex] and apply arctan. If you don't want a negative number in the argument, you can use the fact that arctan is an odd function.
     
  4. Mar 18, 2008 #3
    Thanks Kreizhn,

    I'm now embarrassed.

    this was not seeing the forest for the trees.

    I was about to open Excel and try different values of t until I got it to work out.

    Thanks
    Sparky
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook