Results 1 to 3 of 3

Math Help - Determining exact values for non-special angles

  1. #1
    Newbie
    Joined
    Oct 2010
    Posts
    3

    Determining exact values for non-special angles

    Q: Use an appropriate compound angle formula to determine an exact value for cos(11pi/12)

    I know that you have to express the angle as a sum/difference of two special angles. In this case, cos(165) = cos(45 + 120). I get stuck here. Does this mean I have to break down 120 into (60 + 60)? :/

    any sort of help/hints would be greatly appreciated.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Dec 2009
    Posts
    3,120
    Thanks
    1
    Quote Originally Posted by qswdefrg View Post
    Q: Use an appropriate compound angle formula to determine an exact value for cos(11pi/12)

    I know that you have to express the angle as a sum/difference of two special angles. In this case, cos(165) = cos(45 + 120). I get stuck here. Does this mean I have to break down 120 into (60 + 60)? :/

    any sort of help/hints would be greatly appreciated.
    That would be appropriate, after first writing the identity for Cos\left(45^o+120^o\right).

    You could also write Cos120^o=Cos\left(180^o-60^o\right)=-Cos60^o

    and Sin120^o=Sin60^o having written the compound angle identity for Cos(A+B).
    Last edited by Archie Meade; November 2nd 2010 at 05:44 PM.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by qswdefrg View Post
    Q: Use an appropriate compound angle formula to determine an exact value for cos(11pi/12)

    I know that you have to express the angle as a sum/difference of two special angles. In this case, cos(165) = cos(45 + 120). I get stuck here. Does this mean I have to break down 120 into (60 + 60)? :/

    any sort of help/hints would be greatly appreciated.
    You should know from symmetry the exact values of sin(120) and cos(120).
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Trig Ratios & Special Angles
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: April 14th 2010, 02:17 PM
  2. Trigonometric Ratios and and Special Angles
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: December 2nd 2009, 03:22 AM
  3. Special Angles
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: May 28th 2009, 02:00 PM
  4. trig values of special angles
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: December 15th 2008, 03:51 PM
  5. trigonometric functions of special angles
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: April 21st 2008, 07:26 AM

Search Tags


/mathhelpforum @mathhelpforum