Results 1 to 4 of 4

Math Help - trigo idnentitis question

  1. #1
    Junior Member
    Joined
    Sep 2008
    From
    Australia
    Posts
    72

    Exclamation trigo idnentitis question

    Prove the identity:
    1. sin(A-B)cosB+cos (A-B)sinB=sinA
    2. Prove that sin(∏/4 + A) - sin(∏/4 - A)=Square root 2 sinA
    THANK YOU
    Last edited by sanikui; September 4th 2008 at 06:23 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by sanikui View Post
    Prove the identity:
    sin(A-B)cosB+cos (A-B)sinB=A
    Prove that sin(∏/4 + A) - sin(∏/4 - A)=Square root 2 sinA
    In both cases you should start by using the compound angle formulae:

    sin(A - B) = sin A cos B - sin B cos A
    cos(A - B) = cos A cos B + sin A sin B
    sin(A + B) = sin A cos B + sin B cos A

    I'm assuming there's something missing on the right hand side of the first one .......

    For the second one it also helps to know that \sin \frac{\pi}{4} = \cos \frac{\pi}{4} = \frac{1}{\sqrt{2}}.

    Where are you stuck?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Sep 2008
    From
    Australia
    Posts
    72
    Sorry for write wrong...
    That is sin(A-B)cosB+cos (A-B)sinB=sinA
    Last edited by sanikui; September 4th 2008 at 06:23 PM.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Junior Member
    Joined
    Sep 2008
    From
    Australia
    Posts
    72
    Know do liaw
    1.RHS=(sinAcosB-cosAsinB)cosB+(cosAcosB+sinAsinB)sinB
    =sinAcos^2B+sinAsin^2B
    =sinA(cos^2B+sin^2B)
    =sinA(1)
    =sinA


    THX Mr Fantastic
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. need help for basic trigo question
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: June 19th 2010, 08:47 AM
  2. trigo show question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: August 11th 2009, 07:39 AM
  3. trigo question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: August 10th 2009, 08:29 AM
  4. another trigo question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: December 7th 2008, 12:27 AM
  5. Please help me with this GCSE trigo question
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: October 9th 2008, 03:17 PM

Search Tags


/mathhelpforum @mathhelpforum