it should be 

c:= evalf((a+b)/2):

if not using signum then what should i use .

i tried to do this problem on paper , so it is like this.

f(x)= A*x^2 -B*x -C

let: A=1  , B=2 , C=3

f(0)=-3

f(4)=5

so : a=0        b=4      midpoint=2

      a₁=2       b₁=4     midpoint=3

      a₂=3       b₂=4     midpoint=3.5

      a₃=3.5     b₃=4     midpoint=3.75

      a₄=3.75    b₄=4     midpoint=3.875

      a₅=3.875    b₅=4    midpoint=3.9375

so in my procedure, i want tp assign any values for A,B,C and find the midpoint at this particular a₅ and b₅ where midpoint is 3.9375 which ended with 4 decimal places.