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: Common Trig Identity Derivations

  1. Aug 28, 2011 #1
    I'm looking for a source online that gives the step by step derivation of common trig identities, such as sin(2theta) = 2sin(theta)cos(theta), cos(2theta) = cos2(theta)-sin2(theta), sin2(theta) = (1 - cos(2theta))/2, ect. I did do 20 minuntes or so of searching online, nothing was exactly what I was hoping for. Is there a website explaining the derivations or should I fork out the $100 for a trig text? Thanks
  2. jcsd
  3. Aug 28, 2011 #2


    User Avatar
    Homework Helper

    The simplest way uses the these identities:

    cos(A+B)= cosAcosB - sinAsinB

    and sin(A+B) = sinAcosB + sinBcosA

    then put A=B. Proving the above identities might prove to be a bit more difficult.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook