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Math Help - unsolvable cosine law question.

  1. #1
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    unsolvable cosine law question.

    So i have a triangle with sides 10 and 40, and the angle between them is 70 degrees. I don't know any other measurements of this triangle.

    I use cosine law to find the 3rd side, which turns out to be 37.8.

    Then I found the other 2 angles with sine law, and I obtain 14 degrees and 84 degrees.

    However, the weird thing is the three angles do not add up to 180!!

    HELP
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  2. #2
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    Hello tea123456

    Welcome to Math Help Forum!
    Quote Originally Posted by tea123456 View Post
    So i have a triangle with sides 10 and 40, and the angle between them is 70 degrees. I don't know any other measurements of this triangle.

    I use cosine law to find the 3rd side, which turns out to be 37.8.

    Then I found the other 2 angles with sine law, and I obtain 14 degrees and 84 degrees.

    However, the weird thing is the three angles do not add up to 180!!

    HELP
    The largest angle has a sine equal to 0.9952. But there are two possible angles here: one is 84.4^o, and the other is 180^o-84.4^o. Try the second one instead.

    Grandad
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  3. #3
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    can i ask why there are 2 angles that results the same sine value?

    and how come for 14 degrees, there isnt another angle of 180-14?
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  4. #4
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    Hello tea123456
    Quote Originally Posted by tea123456 View Post
    can i ask why there are 2 angles that results the same sine value?
    I don't know how far your study of trigonometry has gone, but if you're using the Sine and Cosine Rules, you need to know how to handle obtuse angles. The rules are:
    \sin(180^o - \theta)=\sin \theta
    and
    \cos(180^o - \theta)=-\cos \theta
    To understand the reasons for these rules, you'll need to look at the definitions of sine and cosine for angles of any size. These are based on the unit circle. You may find this page helpful.

    and how come for 14 degrees, there isnt another angle of 180-14?
    Because, in a triangle, the smallest angle is always less than 90^o (obviously!) and it is always opposite the shortest side (because of the Sine Rule, where each side is in proportion to the sine of the opposite angle).

    So the 14^o angle is the smallest angle here, because it's opposite the shortest side. And so it's 14^o, not 166^o.

    Grandad
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