trigonometric formula question (15m)

In triangle ABC, a=8cm, b=7cm and c=9cm
a) find the area of traingle ABC
b) Using the result of (a), find the radius of the circumcircle of triangle ABC.

( leave the radical sign in the answer is preferable)

(a)
s=(a+b+c)/2=12

By Heron's formula

A=√s(s-a)(s-b)(s-c)=√(12*4*5*3)=√720

(b)
Let the radius of the circumcircle of triangle ABC is R

2R=abc/2A=252/√720
R
=126/√720
=63/√180
=21/√20
=21√5/10

2008-04-12 15:43:32 補充：
2R=abc/2A 參考 http://en.wikipedia.org/wiki/Law_of_sines