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: Trigonometric Identities Proof

  1. Aug 15, 2009 #1
    1. (A) sin(-x) = - sin x (C) cos(x+y) = cosxcosy - sinxsiny
    (B) cos(-x) = - cos x (D) sin(x+y) = sinxcosy + cosxsiny
    Use these equalities to derive the following trigonometric identities.
    a. absolute value of cos x/2 = [tex]\sqrt{}1 + cosx/2[/tex]
    b. absolute value of sin x/2 = [tex]\sqrt{}1 - cosx/2[/tex]


    3. I'm stuck on these two and tried to think of different ways to solve it but I can't seem to get find a solution to it. Can you please explain how to derive those two equations? Thank You!!
  2. jcsd
  3. Aug 15, 2009 #2


    User Avatar
    Homework Helper

    cos(A+B) = cosAcosB - sinAsinB, try setting A=B and then see what happens. Since there is an x/2, maybe you should replace A with that.
  4. Aug 15, 2009 #3
    (B) cos(-x) = - cos x
    needs to be cos(-x) = cos x without the minus in front on the right side.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook