Math repost  girly93
posted by Writeacher on .
Posted by girly93 on Monday, February 9, 2009 at 2:55am.
i don't understand the sine and cosine rules.
1/2 ab sinC
and cosA= b2 + c2  a2
what???
i have asked my teacher but i still don't get it.

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/websinecostri.pdf