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Homework Help: Compounded Angle Identities

  1. Dec 1, 2008 #1
    1. The problem statement, all variables and given/known data
    cos([tex]\pi[/tex]-x)= -cosX

    the formula is cos(A-b) = cosAcosB+sinAsinB
    so i sub in the given to get..
    cos[tex]\pi[/tex]cosx + sin[tex]\pi[/tex]sinX

    then where do i go from there? im new to math like this, its a much higher level than what im used to, any help would be very apprieciated. thanks.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 1, 2008 #2


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    Homework Helper

    What does cos(pi) and sin(pi) work out to be?
  4. Dec 1, 2008 #3
    sinpi= 0.054803665
    cospi= 0.998497149

    i dont know where to go from there, i know after the dust settles i need to have -cosx somehow.
  5. Dec 1, 2008 #4


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    Staff Emeritus
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    Gold Member

    Those should be values you have memorized....

    (And incidentally, you left your calculator in degree mode on accident)
  6. Dec 1, 2008 #5


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    Homework Helper

    Are you familiar with radians? [tex]\pi =180^o[/tex]
  7. Dec 2, 2008 #6
    You can use both radians and degrees. If you use the radian mode you will write π = 3.141592 and if you use degree mode you will write п = 180o. Anyway, you will get same value. But these values are so easy to remember (even you don't need to remember it, just draw a circle in coordinate system, and remember the x axis is cos and the y axis is sin). Now turn for 180o from 0o and you will get what?

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