# Re: Law of Sines and Cosines

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Posted by Lisa on October 07, 2002 at 02:14:33:

In Reply to: Law of Sines and Cosines posted by Andrew on October 06, 2002 at 21:36:35:

are you trying to find Angle A, that formula you used is to find side a which you already know.
Try cos^(-1) a =(b^2 + c^2 - a^2) /(2bc)
I tried it for the example below and it worked :) remember to set your calc for degrees.
a=5, b=6, c=7
I got this answer ===> 44(degrees) 25'

: The book had an example a=21 b=16.7 c=10.3

: a^2= b^2 + c^2- 2bc cos(A
: (21)^2=(16.7)^2 + (10.3)^2 - 2(16.7)(10.3) cos(A
: 441= 278.89 + 106.09 - 344.02 cos(A
: cos(A= 441 - 278.89-106.09/-344.02
: Cos A = -0.1628
: A=99 (degrees0 22'

: So I substituted a b and c for a=5, b=6, c=7
: So then I got a small number like .776 or something. The answer is 44(degrees) 25'

: What am I doing wrong?

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