To make our estimation more precise, we need to shorten the range which contains the solution. We divide this section into two equal parts: ( a , a/2+b/2) and (a/2+b/2, b). Then we apply the same method to judge whether there is a solution in each section. If there is no solutions in one section( F(a)*F(a/2+b/2) > 0 or F(a/2+b/2)*F(b)> 0), we ignore this section. If there is a solution in one section (F(a)*F(a/2+b/2) < 0 or F(a/2+b/2)*F(b) < 0), we continue to shorten the range until the range is smaller than the error of estimation we expected. Finally, we take the middle point of the final range as a solution of F(x).