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Homework Help: Using the Cosine formula

  1. Apr 2, 2010 #1
    I got this question wrong and I'm hoping someone can explain to me where I went wrong.

    In a triangle...
    angle A = 350
    side b = 7 cm
    side c = 3 cm

    I need to find side a which is the side opposite angle A.

    The formaula to use is -a2 = b2 + c2 - 2bc cos A
    so, with the figures plugged in that comes out as...

    a2 = 72 + 32 - 2 x 3 x 7 cos 35

    which comes to...

    a2 = 58 - 42 cos 35

    SO FAR, I KNOW THAT IS CORRECT. I seem to have made the mistake at this point. I multiplied 16 (58-42) by cos 35 and took the square root of the answer, which gave me 3.62 cm.
    However, apparently the right answer is 4.86 cm.
    What did I do wrong?
     
  2. jcsd
  3. Apr 2, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    58 - 42 cos 35 ≠ (58 - 42) cos35

    58 - 42 cos 35 = 58 - (42 cos35)



    Multiply 42 by cos35, then subtract that from 58.
     
  4. Apr 2, 2010 #3
    That's brilliant! Thanks a lot Doc!
     
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