\[\sin \frac{A}{2}=\sqrt{\frac{(s-b)(s-c)}{bc}} \]
where A is the angle of triangle and s is semiperimeter which is equal to (a+b+c)/2
a , b ,c are the lenghts of triangle ABC
Note that a is the side opposite to the angle A
b is the side opposite to the angle B of the triangle
c is the side opposite to the angle C of the triangle
For a=3 b=4 c=5 s=(a+b+c)/2 = 6
hence Sin(A/2)=0.316
