Results 1 to 3 of 3

Math Help - Sine and Cosine rules question

  1. #1
    Newbie
    Joined
    Nov 2007
    Posts
    14

    Sine and Cosine rules question

    The diagram shows triangle ABC in which AC=4cm, BC=5cm and ACB=60 degrees.

    a) Show that AB = Square root of 21

    b) find the value of sin (angle ABC) in the form k(square root)7 where k is an exact fraction


    Anyone know how to do this?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    Joined
    Oct 2007
    From
    London / Cambridge
    Posts
    591
    For the first make you use the cosine rule

    AB^2 = 4^2 + 5^2 - 2 \cdot 4 \cdot 5 \cdot \cos 60

    can you finish that off ?

    for the second part use the sine rule

    \frac{4}{ \sin ABC} = \frac{ \sqrt {21}}{ \sin 60}

    note \sin 60 = \frac{ \sqrt {3} }{2}
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Nov 2007
    Posts
    14
    yep, thanks bobak.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Question about sine and cosine law
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: February 26th 2011, 08:02 AM
  2. using the sine and cosine rules
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: May 28th 2010, 05:15 PM
  3. Sine and Cosine question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: April 28th 2010, 02:30 AM
  4. Sine and Cosine rules
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: May 26th 2009, 06:14 AM
  5. A question based on the sine & cosine formulas
    Posted in the Trigonometry Forum
    Replies: 5
    Last Post: January 13th 2008, 02:41 PM

Search Tags


/mathhelpforum @mathhelpforum