# Re: Law of Sines and Cosines

[ Follow Ups ] [ Post Followup ] [ Main Message Board ] [ FAQ ]

Posted by Lisa on October 07, 2002 at 13:35:18:

In Reply to: Re: Law of Sines and Cosines posted by Andrew on October 07, 2002 at 07:15:36:

The "new" formula is dervied from the original formula so make sure you understand how it is derived if its not in your book. In your original question you used the old formula wrong, the old forumla is used if you don't know the side and know the angle so that is why you could not use it so you have to remember why, cause it may be on a test. The new formula is derived from the old formula and you have to know how and who.
: I just tryed the new formula and it worked like a charm. Thank you.

:
: : 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?

Name:
E-Mail:

Subject: