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Homework Help: Find exact value of composite trig function.

  1. Oct 5, 2011 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    I began by finding the reference angle of 5pi/4 by subtracting pi, and I got pi/4. pi/4 must be part of an isosceles right triangle, so I drew a diagram with sides 1,1, and √2. I found sin pi/4 to equal 1/√2, which I rationalized and got √2/2. I viewed my cosine chart, and cosine is √2/2 where the angle is pi/4. I answered pi/4 into MyMathLab, which found it incorrect, so I concluded that I am most likely doing it wrong.
  2. jcsd
  3. Oct 5, 2011 #2


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    Homework Helper

    sin(5pi/4) = - 1/√2.

  4. Oct 5, 2011 #3


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    Gold Member

    You might also be able to use the fact that sine and cosine are typically π/2 out of phase, and that cos-1(cos(x))=x.
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