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Math Help - another simple question

  1. #1
    Junior Member lanzailan's Avatar
    Joined
    Mar 2009
    Posts
    27

    another simple question

    Hi people..
    ok, this question is actually very simple and i did it 5 times but i didn't get the answer as provided.

    here is the question..
    A is in 3rd quadrant with cos A=-24/25 and B in the 2nd quadrant with cos B=-8/17. Find tan(A+B).

    Answer:-304/297<<----- i believe that the answer provided is wrong!

    can anybody prove that i'm wrong?
    thanks in advance
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  2. #2
    MHF Contributor
    Grandad's Avatar
    Joined
    Dec 2008
    From
    South Coast of England
    Posts
    2,570

    Trigonometry

    Hello lanzailan
    Quote Originally Posted by lanzailan View Post
    Hi people..
    ok, this question is actually very simple and i did it 5 times but i didn't get the answer as provided.

    here is the question..
    A is in 3rd quadrant with cos A=-24/25 and B in the 2nd quadrant with cos B=-8/17. Find tan(A+B).

    Answer:-304/297<<----- i believe that the answer provided is wrong!

    can anybody prove that i'm wrong?
    thanks in advance
    \tan A = \frac{7}{24}, \tan B = -\frac{15}{8}

    \Rightarrow \tan (A+B) = \frac{\tan A + \tan B}{1 - \tan A \tan B}

    = \left(\frac{7}{24} - \frac{15}{8}\right) \div \left(1 + \frac{7}{24}\times\frac{15}{8}\right)

    = \frac{7\times 8 - 15 \times 24}{24 \times 8 + 7\times 15}

    =-\frac{304}{297}

    Grandad
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  3. #3
    Junior Member lanzailan's Avatar
    Joined
    Mar 2009
    Posts
    27
    ok now i see where i did it wrong..
    in 3rd quadrant tanA should be positive.

    thanks Grandad
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