Posted by **Writeacher** on Monday, February 9, 2009 at 7:01am.

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.

- Math repost - girly93 -
**bobpursley**, Monday, February 9, 2009 at 9:50am
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.

- Math repost - girly93 -
**drwls**, Monday, February 9, 2009 at 9:50am
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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