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: Help with proving a trigonometric identity

  1. Feb 26, 2010 #1
    Hi I've got this problem which has really been bothering me.

    How are you supposed to prove that:
    (Sin[A] Sin[2 A] + Sin[3 A] Sin[6 A])/(Sin[A] Cos[2 A] + Sin[3 A] Cos[6 A]) is identicle to tan[5A].

    I am almost sure that I've got to use the factor formulae, but I've had no luck. Maybe someone can help me please :)
    Thank you for your time!
  2. jcsd
  3. Feb 26, 2010 #2
    Try to "de-factor".
    For instance, Sin(A)Sin(2A)=Sin(2A)Sin(A).
    Recalling the identity that Cos S + Cos T = ?
  4. Feb 26, 2010 #3
    Ok i found out how to work it out. i first multiply by 2 throughout. then i used the factor formulae until i end up with the terms :

    2sin5A sin4A / 2sin4Acos5A which is identicle to sin5A/cos5A which is identicle to tan5A...PROVED!

    The satisfaction of proving an identity or equation is rather incredible!
  5. Feb 26, 2010 #4
    yea! Great sense of satisfaction. you are turning 17 this year?
  6. Feb 26, 2010 #5
    haha indeed i am, still young i guess, just scratching the surface of mathematics. The subject really gets you thinking
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook