Question

area of a triangle in terms of in radius and circumradius

Answer 1

Let △ABC be a triangle whose sides are of lengths a,b,c.

Then the area A of △ABC is given by:
A=abc/4R
where R is the circumradius of △ABC.

Answer 2

for proof follow this link
http://www.proofwiki.org/wiki/Area_of_Triangle_in_Terms_of_Circumradius