Question: i need help please

i have a procedure like this 

>f:=x-> A*x^2 - B*x -C:
   Bisection:= proc(f,a,b)
   local c;
   if abs(evalf(b-a))<=0 then return c; end if;
   if signum(f(c))=signum(f(a)) then
   end if;
   end proc;

> Bisection(1,-2,-3,0,4);


this procedure only give me midpoint of the initial value between a=0 and b=4, but if i want to find the midpoint between 

a5  and b5 then what should i do.



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