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Math Help - Stupid simple sine/cosine problem

  1. #1
    Junior Member Fnus's Avatar
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    Stupid simple sine/cosine problem

    Alright, so I have this right-angled triangle and I thought I did it right, but apparently I didn't, so any help would be great appreciated!

    I wanted to find B,
    So I just used b / SinB = c /sinC

    SinB = b / c

    SinB = sin^-1(38/63)

    I end up with 37.1, but the answers said I wanted 52.9!
    So I switched the sin^-1 for a cos^-1, and I ended up with 52.9. ):

    Now my question is, why do I want to use cos instead of sin?
    I really don't get it! I can't see why it's wrong to use sine since since is Opposite/Hypothenuse.

    Thanks in advance!
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  2. #2
    MHF Contributor
    skeeter's Avatar
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    Quote Originally Posted by Fnus View Post
    Alright, so I have this right-angled triangle and I thought I did it right, but apparently I didn't, so any help would be great appreciated!

    I wanted to find B,
    So I just used b / SinB = c /sinC

    SinB = b / c

    SinB = sin^-1(38/63)

    I end up with 37.1, but the answers said I wanted 52.9!
    So I switched the sin^-1 for a cos^-1, and I ended up with 52.9. ):

    Now my question is, why do I want to use cos instead of sin?
    I really don't get it! I can't see why it's wrong to use sine since since is Opposite/Hypothenuse.

    Thanks in advance!
    based on your sketch (sure it's right?), you are correct ...

    m\angle{B} = \sin^{-1}\left(\frac{38}{63}\right) = 37.1^{\circ}

    m\angle{A} = \cos^{-1}\left(\frac{38}{63}\right) = 52.9^{\circ}
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  3. #3
    Junior Member Fnus's Avatar
    Joined
    Nov 2006
    Posts
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    Yeah, I triple checked and more. ):

    Well, thanks for clearing it up at least, because I really couldn't see what I'd done wrong.
    I guess I'll just have to ask my teacher tomorrow!

    Thanks a lot for replying. (:
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