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Thread: An 'undefined' answer with sine rule [non-right angled triangles]

  1. #1

    Exclamation An 'undefined' answer with sine rule [non-right angled triangles]

    Hi. So I just created a question for myself regarding the sine rule for non-right angled triangles and something odd came up. I just needed to do one last thing to get the answer and when I did it, it ended up being 'undefined'. Here is the working out:

    Angle C=62 [degrees]
    Side c= 7cm
    Side b = 8cm

    Find angle B:

    sin(A)/a=sin(B)/b=sin(C)/c

    sin(62)/7=sin(B)/8
    (sin(62)/7)*8=sin(B)
    sin^-1((sin(62)/7)*8)=B

    And when I do that step it says undefined. I tried doing sin(62)/7 first, then doing that *8, then putting all that within the brackets of sin^-1()

    Nothing has worked so far... Please help!!!

  2. #2
    Elite Member mmm4444bot's Avatar
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    Google arcsin((sin(62)/7)*8) and you'll get its value: -1.00615005
    "English is the most ambiguous language in the world." ~ Yours Truly, 1969

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    Quote Originally Posted by kuku9017 View Post
    Hi. So I just created a question for myself regarding the sine rule for non-right angled triangles and something odd came up. I just needed to do one last thing to get the answer and when I did it, it ended up being 'undefined'. Here is the working out:

    Angle C=62 [degrees]
    Side c= 7cm
    Side b = 8cm

    Find angle B:

    sin(A)/a=sin(B)/b=sin(C)/c

    sin(62)/7=sin(B)/8
    (sin(62)/7)*8=sin(B)
    sin^-1((sin(62)/7)*8)=B

    And when I do that step it says undefined. I tried doing sin(62)/7 first, then doing that *8, then putting all that within the brackets of sin^-1()

    Nothing has worked so far... Please help!!!
    You've found that sin(B)=1.00908296. There is no angle with this sine, so you've shown that there is no solution -- no such triangle exists.

    Try drawing the triangle, and see what happens!

    (By the way, Google takes 62 to be radians, which is why it doesn't show an error.)

  4. #4
    Elite Member mmm4444bot's Avatar
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    Quote Originally Posted by kuku9017 View Post
    [degrees]
    I missed that.

    I think it's a good idea to use a degree symbol, when writing angle measurements in degrees, but I understand a lot of instructors don't do that.

    sin(62)/7=sin(B)/8

    (sin(62)/7)*8=sin(B)

    sin^-1((sin(62)/7)*8)=B
    Last edited by mmm4444bot; 01-30-2018 at 03:16 PM.
    "English is the most ambiguous language in the world." ~ Yours Truly, 1969

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