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Posted by 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 - , 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 - , 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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