\[\cos \frac{A}{2}=\sqrt{ \frac{s(s-a)}{bc}} \]
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 we get s=6
Cos(A/2)=0.948
