1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Trig identity proof

  1. Jul 13, 2012 #1
    1. The problem statement, all variables and given/known data

    Prove the identity.


    2. Relevant equations

    http://postimage.org/image/vjhwki1ax/ [Broken]

    3. The attempt at a solution

    http://s13.postimage.org/jkhubi4lz/DSC03534.jpg
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 13, 2012 #2

    Mark44

    Staff: Mentor

    Speaking only for myself, I would be more inclined to help out if I didn't have to open one link to see the problem, and open another link to see what you did.
     
  4. Jul 13, 2012 #3
    I noticed that after all of your work, you got the problem to cos(3x)cos(x)-sin(x)sin(3x).

    Using sec(x) = 1/cos(x) and csc(x) = 1/sin(x);

    cos(3x)/sec(x) - sin(x)/csc(3x)
    cos(3x)/(1/cos(x)) - sin(x)/(1/sin(3x))
    cos(3x)cos(x) - six(x)sin(3x)
     
  5. Jul 13, 2012 #4

    Curious3141

    User Avatar
    Homework Helper

    I can't view his solution. But if he's already got it into that form, he can just use the angle sum formula for cosine to express that as [itex]\cos kx[/itex], where k is some positive integer (which he needs to work out). Then use the double angle formula for cosine to split it up again, yielding the required proof.
     
  6. Jul 13, 2012 #5

    eumyang

    User Avatar
    Homework Helper

    Looking at the OP's solution, the OP went the complicated route to go from the LHS to the bolded part above. Villyer just simplified the process.
     
  7. Jul 13, 2012 #6
    cos(3x)cos(x) - six(x)sin(3x)

    cos (4x)

    cos^2 2x - sin^2 2x

    Thanks all.
     
  8. Jul 13, 2012 #7

    Curious3141

    User Avatar
    Homework Helper

    Looks great, but you might want to put "=" signs in between the lines.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Trig identity proof
  1. Trig identity proof. (Replies: 2)

  2. Trig Identity proof (Replies: 12)

  3. Trig Identity proof (Replies: 2)

Loading...