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Homework Help: Trigonometric Identities

  1. Mar 3, 2010 #1
    1. The problem statement, all variables and given/known data
    Without using tables(calculators) find the numerical value of

    Sin[Pi/8]^2 - Cos[3 (Pi/8)]^4

    2. Relevant equations

    3. The attempt at a solution
    I tried changing it to:
    1-cos[pi/8]^2 - cos[3pi/8]^4 but have no idea where to go... its really got me scratching my head.

    Any ideas? Any help is much appreciated, thank you.
  2. jcsd
  3. Mar 3, 2010 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi kenshaw93! Welcome to PF! :smile:

    (have a pi: π and try using the X2 tag just above the Reply box :wink:)

    You know what cos and sin of π/4 are …

    so use one of the standard trigonometric identities to get to π/8 and 3π/8. :smile:
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