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Math Help - Sloving triangle with law of sine and cosine

  1. #1
    Member M670's Avatar
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    Sloving triangle with law of sine and cosine

    Solve the missing angles
    a=15cm b=18cm c=30cm
    so If I use  a^2=b^2+c^2-2bc\cdot\cos A
    i get angle A is 42.12 degrees but then B and C don't work out i know I can use the law of sine once I have one angle but I am just asking why doesn't B and C work with the law of cosine?
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  2. #2
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    Re: Sloving triangle with law of sine and cosine

    \cos{A} = \frac{b^2+c^2-a^2}{2bc}

    \cos{B} = \frac{a^2+c^2-b^2}{2ac}

    \cos{C} = \frac{a^2+b^2-c^2}{2ab}
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    Member M670's Avatar
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    Re: Sloving triangle with law of sine and cosine

    Quote Originally Posted by skeeter View Post
    \cos{A} = \frac{b^2+c^2-a^2}{2bc}

    \cos{B} = \frac{a^2+c^2-b^2}{2ac}

    \cos{C} = \frac{a^2+b^2-c^2}{2ab}

    Skeeter when I Calculated it for cos B  \frac{325-225+900}{900} = \frac{999}{900} =1.11
    and then but my answer should be between 0 and 1 and this doesn't work why?
    I see my mistake I think
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    Re: Sloving triangle with law of sine and cosine

    look at the signs in the numerator ...
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  5. #5
    Member M670's Avatar
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    Re: Sloving triangle with law of sine and cosine

    Quote Originally Posted by M670 View Post
    Skeeter when I Calculated it for cos B  \frac{325-225+900}{900} = \frac{999}{900} =1.11
    and then but my answer should be between 0 and 1 and this doesn't work why?
    I see my mistake I think
    Its -324+225+900/900 =.81 not 1.11
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    Re: Sloving triangle with law of sine and cosine

    It helps to construct a triangle of 1/2 size but label sides as given

    For the obtuse angle cos A = (15^2 + 18^2 - 30^2) / 2*15*18 =-0.65
    A= 130.5
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  7. #7
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    Re: Sloving triangle with law of sine and cosine

    Its -324+225+900/900 =.81 not 1.11www.louisvuittonend.com
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