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Homework Help: Solving problems using Half Angle identities

  1. Nov 19, 2008 #1
    1. The problem statement, all variables and given/known data

    Solve for x using half-angle identities

    cos (x/2) = -√2 / 2

    2. Relevant equations

    cos(x/2) = ± √(1+cosx)/2

    3. The attempt at a solution

    I am trying to figure out what to do with the identity, but I have no idea how to start. I know that x = 270 degrees or 3pi/2, but I do not know how to get there. Can someone head me in the right direction?
    Last edited: Nov 19, 2008
  2. jcsd
  3. Nov 19, 2008 #2


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    Science Advisor

    cos2u= (1+ cos 2u)/2

    Let u= x/2. Then [itex](1+ cos x)/2= (-\sqrt{2}/2)^2= 1/2[/itex] so 1+ cos(x)= 0, cos(x)= -1.
  4. Nov 19, 2008 #3
    Thank you so much
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