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: Summing cosines of different amplitude

  1. Mar 8, 2008 #1
    Hi there

    I am trying to sum many cosines of different amplitude and phase shift, but same ang. frequency (it's not a coursework question). My first thoughts are to sum them two at a time (to simplify matters?), probably using complex numbers. I tried doing it symbolically in MATLAB but it wasn't able to simplify things. Supposing the ang. frequency is 1, I know that the solution can be written:

    C\cos (t + \delta ) + D\cos (t + \varepsilon ) = E\cos (t + \varphi)}

    where I would have to solve for E and phi. Or equivalently:

    \displaystyle{Ae^{i(t + \delta )} + e^{i(t + \varepsilon )} = Be^{i(t + \varphi )}}

    where I would have to solve for B and phi.

    Then I split things into two equations (one using real part/cosines, other using imag. part/sines), and eliminate B. Unfortunately this approach doesn't seem to help, as I just end up with a messy arctan of sums of sines and cosines (of different amplitudes -- i.e. back to original problem!).

    Any suggestions as to a more fruitful approach?

    Thank you.
  2. jcsd
  3. Mar 8, 2008 #2
    Ok, nevermind, got it figured out! Just used a bit of geometry
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook