Posted by Writeacher on Monday, February 9, 2009 at 7:01am.
Your question makes no sense to me.
The law of cosines is this
a^2=b^2+c^2 - 2bc * CosA
I have no idea what 1/2 ab SinC is , unless it is the area formula for a triangle. You need to be more specific.
The law of cosines, in one form, is
a^2 = b^2 + c^2 - 2bc cos A
The law of sines is
a/sin A = b/sinB = c/sinC
= 2R
Your cosA equation is incorrect.
(1/2) a b sin C is a formula for the area of a triangle, and has nothing to do with the law of sines.
For more about these equations, see
http://www.mathcentre.ac.uk/resources/workbooks/mathcentre/web-sinecostri.pdf
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