double findRootBetween(double l, double u){
 double res;
 assert(l <= u); assert(!sameSign(f(l), f(u))); 
 while (!isSmallNumber(f(middleOf(l,u)))){ 
   if(sameSign(f(middleOf(l,u)), f(u)))
     u = middleOf(l,u);
   else 
     l = middleOf(l,u);
 }
 res = middleOf(l,u);
 assert(isSmallNumber(f(res)));
 return res;
}