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Help w/ these proofs!

  1. Jan 2, 2012 #1
    I'm having trouble with these two proofs.
    cos(n∏+θ)=(-1)^n cos θ
    ln|sec x|= -ln|cos x|

    I know for the first one that I have to incorporate log somehow but that's about all I got from it.
     
  2. jcsd
  3. Jan 2, 2012 #2
    For the second one, think about the laws of logarithms you know.
     
  4. Jan 2, 2012 #3
    For the first one i assume the righthandside means ((-1)^n)cos(theta), how can you expand the left hand side, do you know of any identites or formulae?

    I'm pretty sure you don't need log for the first one.
     
  5. Jan 2, 2012 #4
    For the first one, start by thinking about how [itex]cos(\pi+\theta)[/itex] is related to [itex]cos(\theta)[/itex] - the case where [itex]n=1[/itex]

    http://www.cliffsnotes.com/study_guide/Addition-Identities.topicArticleId-11658,articleId-11610.html [Broken]
     
    Last edited by a moderator: May 5, 2017
  6. Jan 2, 2012 #5

    SammyS

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    Use the angle addition identity for the cosine:
    cos(A+B)=cos(A)cos(B)-sin(A)sin(B)​
     
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