Question

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.

Answer 1

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.

Answer 2

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