1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook