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Math Help - Am I doing something wrong?

  1. #1
    Member
    Joined
    Sep 2007
    Posts
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    Unhappy Am I doing something wrong?

    Algebra II: Equations and Triangles - Math for Morons Like Us

    Scroll down the page until you hit the part that says Law of Cosines.
    I have been trying to solve that problem for hours now and I don't get the same answer for Angle A.

    Here is the problem.

    --------------------------------------------------------------
    1. Problem: In triangle ABC, a = 24,
    c = 32, and angle B = 115o.
    Solve the triangle.

    Solution:
    We know two sides. Find the third
    using the law of cosines.

    b2 = a2 + c2 - 2ac(cos B)

    Plug in any known information.

    b2 = 242 + 322 - 2 * 24 * 32(-.4226)
    b2 = 2249

    Solve for b by taking the square
    root of each side.

    b = SQRT(2249) = 47.4

    Using the law of sines,
    angle A = 27.32o.

    Since we know that triangles are made of
    three angles that sum to 180o,
    we can find the measure of angle C by
    setting up an equation.

    Angle C = 180o - 115o - 27.32o = 37.68
    ----------------------------------------------------------------

    After reaching the part where is says.
    Using the law of sines,
    angle A = 27.32o.
    I am unable to get angle A = 27.32 I keep getting A = 29.55

    Here is the formula I keep using.

    (24(Sin115))/(Sin(47.4)) = 29.55

    I don't get how the answer is 27.32

    Can anyone please tell me if I am doing something wrong or if the site is wrong...

    (Special request: Does anyone know of a good site like the one I posted with step by step instructions on Trigonometry and such.)
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  2. #2
    MHF Contributor
    Joined
    Apr 2005
    Posts
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    Quote Originally Posted by Brazuca View Post

    Here is the formula I keep using.

    (24(Sin115))/(Sin(47.4)) = 29.55

    I don't get how the answer is 27.32

    Can anyone please tell me if I am doing something wrong or if the site is wrong...
    I thought 47.4 is distance? There is no sine value for a distance.
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  3. #3
    Member
    Joined
    Sep 2007
    Posts
    86

    Question ???

    I understand now, what I had to do.

    (24(Sin115))/(47.4)) = .45889

    Then Sin^-1(.45889) = 27.3

    Thanks.
    Last edited by Brazuca; July 19th 2008 at 09:19 AM.
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