Question
Fri February 13, 2009 By:

triangles

Expert Reply
Sat February 14, 2009

A,B and C are the interior angles of a triangle.

So,

 A +  B+  C= 180

Now,

sin[( B+C)/2]=sin [(180-A)/2]

=sin[90-(A/2)]

=cos(A/2)

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