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 identity

  1. Nov 27, 2008 #1
    Why is [tex]cos^2 x = \frac{1}{2} + \frac{cos(2x)}{2}[/tex]

  2. jcsd
  3. Nov 27, 2008 #2


    User Avatar
    Homework Helper

    why is
    why is
  4. Nov 27, 2008 #3


    User Avatar
    Science Advisor

    Starting where? Do you know cos(a+ b)= cos(a)cos(b)- sin(a)sin(b)? If so then take a= b= x. If not then what definition of cos(x) are you using so that you can get that?
  5. Nov 27, 2008 #4
    I think this is what you wanted to know:
    Since cos(2x)=cos2x-sin2x
    cos2x=1/2 + cos(2x)/2
  6. Nov 27, 2008 #5

    [tex]\frac{1}{2} + \frac{cos(2x)}{2}[/tex]





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