Results 1 to 2 of 2
Like Tree1Thanks
  • 1 Post By ebaines

Math Help - Trig Problem

  1. #1
    Newbie
    Joined
    Nov 2013
    From
    Dar es Salaam
    Posts
    1

    Trig Problem

    Hi All,
    for a triangle ABC, angle B is 50 degrees. Side AB is twice that of AC. Find angle A.

    This means that side AB= x and AC= 2x. Then I thought the only way I could find angle A is if i could find x and then use the Sine rule to find A. But I don't have side AB? Can I assume this is 3x...?

    Using the Cosine rule equation: (2x)^2 = (x)^2 + (3x)^2 - 2(x)(3x) Cos50
    I continue with this to solve for x...?

    Thanks
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor ebaines's Avatar
    Joined
    Jun 2008
    From
    Illinois
    Posts
    1,069
    Thanks
    298

    Re: Trig Problem

    I think you must have a typo in your problem. If AB=x and AC=2x, then from the law of sines you have:

     \frac { \sin (C)} {2x} = \frac { \sin (B)} {x}

     \sin(C) = 2 \sin(50) = 1.532

    which is impossible. I wonder if the problem statement should have said: "Side AB is twice that of BC?" If so, then let length BC = x, AB=2x, and AC = y, and from the law of sines you have:

     \frac { \sin (A)} {x} = \frac { \sin (50)} {y}

    and from law of cosines:

     y = \sqrt {x^2 + (2x)^2 - 4x^2 \cos (50) } = x \sqrt { 5-4 \cos 50}

    Sub this back into the previous equation and solve for  \sin A
    Last edited by ebaines; November 21st 2013 at 12:29 PM.
    Thanks from BobbyBali
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 2
    Last Post: March 20th 2012, 05:32 PM
  2. Trig word problem - solving a trig equation.
    Posted in the Trigonometry Forum
    Replies: 6
    Last Post: March 14th 2011, 07:07 AM
  3. Replies: 3
    Last Post: January 2nd 2011, 08:20 PM
  4. Trig problem
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: March 3rd 2010, 09:13 AM
  5. Trig Problem
    Posted in the Trigonometry Forum
    Replies: 6
    Last Post: December 21st 2009, 08:23 PM

Search Tags


/mathhelpforum @mathhelpforum